unix: Refactor GTK bindings from SDL_tray to separate SDL_gtk module to allow shared usage

- SDL creates and requires usage of a specific glib context. This context is set as the global
  glib context with SDL_Gtk_EnterContext and the previous context is restored with
  SDL_Gtk_ExitContext.

- To avoid changing the behavior of SDL_tray, which is the only consumer of SDL_gtk currently,
  the SDL_UpdateTray function now unconditionally runs a single glib frame iteration and is
  responsible for dispatching glib events for all consumers in SDL_PumpEvents.

- Cleaned up some error handling in SDL_tray.

src/SDL.c

@@ -30,6 +30,10 @@
 // this checks for HAVE_DBUS_DBUS_H internally.
 #include "core/linux/SDL_dbus.h"
 
+#ifdef SDL_PLATFORM_UNIX
+#include "core/unix/SDL_gtk.h"
+#endif
+
 #ifdef SDL_PLATFORM_EMSCRIPTEN
 #include <emscripten.h>
 #endif
@@ -663,6 +667,10 @@ void SDL_Quit(void)
     SDL_DBus_Quit();
 #endif
 
+#ifdef SDL_PLATFORM_UNIX
+    SDL_Gtk_Quit();
+#endif
+
     SDL_QuitTimers();
     SDL_QuitAsyncIO();