switch to GtklLabel for statusbar buttons + remove non nedded resize-grip code

- fixes the large statusbar with GTK3
- remove extra resize-grip code, which is handle by gtk+ itself
GTK2 builds displays the resize-grip without the extra code.
Since GTK+-3.14 the rezise-grip widget is removed.
- the size of the statusbar is now themable

pluma/pluma-window.c

@@ -329,9 +329,6 @@ pluma_window_window_state_event (GtkWidget           *widget,
 
 		show = !(event->new_window_state &
 			(GDK_WINDOW_STATE_MAXIMIZED | GDK_WINDOW_STATE_FULLSCREEN));
-
-		_pluma_statusbar_set_has_resize_grip (PLUMA_STATUSBAR (window->priv->statusbar),
-						      show);
 	}
 
 	return FALSE;