QPA platform probing

Status:

• QT_QPA_PLATFORM only supports a single value
• QPA plugin failing brings down the whole application
• this is problematic on modern platforms (Wayland, Mir) that provide legacy support (XWayland, XMir)

Prior art

• GTK supports a comma-separated list (since 3.10) in GDK_BACKENDS, trying them out in order https://developer.gnome.org/gtk3/3.10/GtkSettings.htm
  • now tries Mir before X11 https://git.gnome.org/browse/gtk+/commit/?h=gtk-3-22&id=05a060c6a6f6762ede5392fa9a763034ca9140c1
• SDL probes backends in a hardcoded order, overridable by SDL_VIDEODRIVER http://sdl.beuc.net/sdl.wiki/SDL_envvars
  • tries X11 first, we're working to change that

Proposal

• add (optional?) API for QPA plugins probing the system
• have QT_QPA_PLATFORM accept a comma-separated list
• alternatively a higher-priority QT_QPA_PLATFORMS to not modify behaviour
• have QApplication try the plugins in order (skipping any missing ones)
• default order on Linux should have Wayland and Mir prior to X11