Tab loading was relying on replacing the contents of '#content'
with the loading indicator, then replacing it with the loading tab
content and blocking rendering until the tab was ready by not
yielding. This is problematic for tabs that load some data
asynchronously, like PID and OSD.
Instead, put the loading indicator in front of everything else
and load new content inside '#content' next to the loading indicator
(but without showing it). Once the content and data are fully loaded
we fade out the loading indicator with a 0.4s long animation and
then we remove. This works for both synchronous and asynchonous
loading of tabs.
flasher.
The welcome (aka 'landing') and firmware flasher were already modelled
as tabs but were never displayed as tabs.
The UI didn't make sense when the whether the welcome message or
firmware flasher was displayed since there was no active tab or other
indication.
