Add wxDisplay::GetDepth() and use it for wxDisplayDepth()

Allow getting the depth of any display, not just the primary one, even
though this is not implemented for Unix ports currently.

Mostly do this for consistency with the other display-related functions.

src/gtk/utilsgtk.cpp

@@ -95,15 +95,6 @@ bool wxColourDisplay()
     return true;
 }
 
-int wxDisplayDepth()
-{
-#ifdef __WXGTK4__
-    return 24;
-#else
-    return gdk_visual_get_depth(gdk_window_get_visual(wxGetTopLevelGDK()));
-#endif
-}
-
 wxWindow* wxFindWindowAtPoint(const wxPoint& pt)
 {
     return wxGenericFindWindowAtPoint(pt);