Updates and feature additions to failsafe system.

- Added failsafe flightmode and rc control box.

To make failsafe procedure a separate flight mode and make it possible
to trigger failsafe with an AUX switch.

- Failsafe mode is activated when failsafe is active.

RC link lost is simulated with the failsafe AUX switch.
When NOT armed: failsafe switch to failsafe mode is shown in GUI (mode
tab).

- Activate failsafe mode with AUX switch.

- Prevent arming when failsafe via AUX switch is active (safety issue).

- Make failsafe disarm if motors armed and throttle was LOW (2D & 3D)
for `failsafe_throttle_low_delay` time (__JustDisarmEvent__).

Applied code changes to effectively add pull request: Make failsafe
disarm if motors armed and throttle low #717.

- Use failsafeIsMonitoring() to actually start monitoring.

- Added `failsafe_kill_switch` to code.

When set to 1 (0 is default), the failsafe switch will instantly disarm
(__KillswitchEvent__) instead of executing the landings procedure.
Arming is NOT locked after
this, so the craft could be re-armed if needed.
This is intended for racing quads where damage and danger must be
minimized in case of a pilot error.

- Added `failsafe_throttle_low_delay`, adapted documentation.

Used to adjust the time throttle level must have been LOW
to _only disarm_ instead of _full failsafe procedure_
(__JustDisarmEvent__).

- Updated the failsafe documentation.

- Re-enable arming at end of failsafe procedure.

At the end of a handled failsafe event, that means: auto-landing,
__JustDisarmEvent__ or __KillswitchEvent__, the RX link is monitored for
valid data.
Monitoring is a part of the failsafe handling, which means the craft is
still in failsafe mode while this is done.
Arming is re-enabled (allowed) when there is a valid RX link for more
then XX seconds, where XX depends on the handled event like this:
1. XX = 30 seconds after auto landing.
2. XX = 3 seconds after __JustDisarmEvent__.
3. XX = 0 seconds after __KillswitchEvent__.

NOTE: When armed via an AUX switch, you will have to switch to the
disarmed position at the very end to be able to re-arm.
The failsafe mode will not end until you do.

- __KillswitchEvent__ has now priority over __JustDisarmEvent__

- Apply rxfail values instantly when failsafe switch is ON

- Added missing cases to display.c

Show M when failsafe is monitoring for RX recovery (AND disarming when
armed with a switch).

===

Reworked the code from counter-based to time-based.

- AUX failsafe switch now has identical behavior to RX loss.

- Added RX failure and RX recovery timing.

- __KillswitchEvent__ skips RX failure detection delay (direct disarm).

===

[UNIT TESTS]

Adapted failsafe related unittests from counter-based to time-based

- Added failsafeOnValidDataFailed() to some tests

- Removed duplicate test setup from rc_controls_unittest.cc

- Removed magic numbers from rx_ranges_unittest.cc and rx_rx_unittest.cc

- Reworked all test-cases for flight_failsafe_unittest.cc

src/main/io/serial_msp.c

@@ -131,7 +131,7 @@ void useRcControlsConfig(modeActivationCondition_t *modeActivationConditions, es
 #define MSP_PROTOCOL_VERSION                0
 
 #define API_VERSION_MAJOR                   1 // increment when major changes are made
-#define API_VERSION_MINOR                   12 // increment when any change is made, reset to zero when major changes are released after changing API_VERSION_MAJOR
+#define API_VERSION_MINOR                   13 // increment when any change is made, reset to zero when major changes are released after changing API_VERSION_MAJOR
 
 #define API_VERSION_LENGTH                  2
 
@@ -346,6 +346,7 @@ static const box_t boxes[CHECKBOX_ITEM_COUNT + 1] = {
     { BOXSERVO2, "SERVO2;", 24 },
     { BOXSERVO3, "SERVO3;", 25 },
     { BOXBLACKBOX, "BLACKBOX;", 26 },
+    { BOXFAILSAFE, "FAILSAFE;", 27 },
     { CHECKBOX_ITEM_COUNT, NULL, 0xFF }
 };
 
@@ -698,6 +699,10 @@ void mspInit(serialConfig_t *serialConfig)
     }
 #endif
 
+    if (feature(FEATURE_FAILSAFE)){
+        activeBoxIds[activeBoxIdCount++] = BOXFAILSAFE;
+    }
+
     memset(mspPorts, 0x00, sizeof(mspPorts));
     mspAllocateSerialPorts(serialConfig);
 }
@@ -819,7 +824,8 @@ static bool processOutCommand(uint8_t cmdMSP)
             IS_ENABLED(IS_RC_MODE_ACTIVE(BOXAUTOTUNE)) << BOXAUTOTUNE |
             IS_ENABLED(FLIGHT_MODE(SONAR_MODE)) << BOXSONAR |
             IS_ENABLED(ARMING_FLAG(ARMED)) << BOXARM |
-            IS_ENABLED(IS_RC_MODE_ACTIVE(BOXBLACKBOX)) << BOXBLACKBOX;
+            IS_ENABLED(IS_RC_MODE_ACTIVE(BOXBLACKBOX)) << BOXBLACKBOX |
+            IS_ENABLED(FLIGHT_MODE(FAILSAFE_MODE)) << BOXFAILSAFE;
         for (i = 0; i < activeBoxIdCount; i++) {
             int flag = (tmp & (1 << activeBoxIds[i]));
             if (flag)