ui.backend.gtk: set the source priority to idle, fixes #1879

basis/ui/backend/gtk/gtk.factor

@@ -77,6 +77,7 @@ M: gtk-clipboard set-clipboard-contents
 :: with-timer ( quot -- )
     <timer-funcs> &free
     GSource heap-size g_source_new &g_source_unref :> source
+    source G_PRIORITY_DEFAULT_IDLE g_source_set_priority
     source f g_source_attach drop
     [ quot call( -- ) ]
     [ source g_source_destroy ] [ ] cleanup ;