Configure toolkits to use their Wayland backends by default

As people will be installing apps that use other toolkits than GTK on their phones, it would be nice to have them at least trying to do reasonable thing regarding Wayland/X11 backend selection, as it's often the only thing that makes the whole difference between the app being usable or completely broken.

For SDL2:

SDL_VIDEODRIVER=wayland

For Qt:

QT_QPA_PLATFORM=wayland
QT_WAYLAND_DISABLE_WINDOWDECORATION=1

A tricky thing with Qt will be to install its Wayland backend (qtwayland5 package) automatically when installing Qt, as this doesn't happen by default in Debian.

If it's possible, it would be also nice to have libglfw3-wayland package being automatically preferred when some dependency pulls GLFW in (it replaces the X11 libglfw3 one); also, Allegro will need to be handled in some way to make Animatch packagable as deb (likely with similar approach to GLFW one, creating a set of liballegro5.2-sdl packages).