wayland: Move out popup logic to its own file

We'll want to expose popup logic outside of meta-wayland-pointer.c and
one day we'll also probably want to add touch support for popups, so
lets move it to its own file. There are no significant semantical
changes, only refactoring.

https://bugzilla.gnome.org/show_bug.cgi?id=744452

src/wayland/meta-wayland-types.h

@@ -26,6 +26,8 @@ typedef struct _MetaWaylandSeat MetaWaylandSeat;
 typedef struct _MetaWaylandPointer MetaWaylandPointer;
 typedef struct _MetaWaylandPointerGrab MetaWaylandPointerGrab;
 typedef struct _MetaWaylandPointerGrabInterface MetaWaylandPointerGrabInterface;
+typedef struct _MetaWaylandPopupGrab MetaWaylandPopupGrab;
+typedef struct _MetaWaylandPopup MetaWaylandPopup;
 typedef struct _MetaWaylandKeyboard MetaWaylandKeyboard;
 typedef struct _MetaWaylandTouch MetaWaylandTouch;
 typedef struct _MetaWaylandDataSource MetaWaylandDataSource;