[BUG] Steam overlay injection on Linux.

[Alia5]

Bug Description

Currently, on Linux, Steam overlay injection is broken when SISR is not launched via Steam.
Thus, SISR will always ask the user to enable continuous drawing, unless this option is specified already.

Context

SISR, when not launched via Steam, attempts to load the Steam Overlay at runtime, after it has found a "marker shortcut" within Steams non-Steam game list.
SISR requires doing this, when not launched via Steam, to:

• get the actual Steam Virtual gamepads
• get a Steam Input config
  (otherwise it would just use Steams Desktop-Config, with no option to assing "gamepad" controls
• Display Steams touch/radial-menus (if the --wcd option was provided)
• Have Steam create the "CEF Tab" where
  SISR injects a tiny bit of Javascript to get a callback when the Steam Overlay is opened/closed

Most of this works on Linux already, however the actual "Overlay" parts to do not. (Display Steams touch/radial-menus, have Stem create the "CEF Tab".

On Windows this works mostly fine by just setting a few environment variables, loading the GameOverlayRenderer.dll and Steam / the GameOverlayRenderer will magically setup up the rest...

[Alia5]

In Gaming Mode (SteamDeck), there is no specific overlay-tab.
We can work around this by injecting the overlay-state notifier in the "Big Picture" MainMenu*_-Tab and query the browsers window-focus.

See: 874daf4

This makes this at least partially fixed.