core: Add a copy of GDK_PRIORITY_EVENTS

Quoting Carlos:
The META_PRIORITY_EVENTS ± 1 happening below are in order to set these idles
and timeouts in a priority that is relative to the literal GDK event priority,
making those diverge is a likely way to sneakily break things.
But that's unlikely to happen, and decoupling mutter from GTK further
should make it moot, so perhaps it's alright after all.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2407>

src/core/events.h

@@ -25,6 +25,14 @@
 
 #include "meta/display.h"
 
+/**
+ * META_PRIORITY_EVENTS: (value 0)
+ *
+ * This is the priority that events from the X server are given in the main loop.
+ */
+#define META_PRIORITY_EVENTS    (G_PRIORITY_DEFAULT)
+
+
 void meta_display_init_events (MetaDisplay *display);
 void meta_display_free_events (MetaDisplay *display);