Upgrade SDL to 3.6 and use native D-Bus tray backend

[mgerb]

Upgrade SDL to 3.6 when released. This should give us the native DBus tray backend so that we can remove GTK/Ayatana AppIndicator.

Could potential use 3.5 when it's released if we don't want to wait for the stable version (3.6).

See the PR here: libsdl-org/SDL#15189