ibus-ui-gtk3: Draw gray color on PropertyPanel handle

Users can move the position of IBus PropertyPanel with the mouse
but currently it is too hard to find the handle on the panel.
Now the handle is drawn by the gray color for the visibility.

TEST=ui/gtk3/ibus-ui-gtk3

Review URL: https://codereview.appspot.com/219520043

ui/gtk3/handle.vala

@@ -2,7 +2,7 @@
  *
  * ibus - The Input Bus
  *
- * Copyright(c) 2011 Peng Huang <shawn.p.huang@gmail.com>
+ * Copyright(c) 2011-2015 Peng Huang <shawn.p.huang@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -36,6 +36,19 @@ class Handle : Gtk.EventBox {
                              Gdk.EventMask.BUTTON1_MOTION_MASK;
         set_events(mask);
         m_move_begined = false;
+
+        // Currently it is too hard to notice this Handle on PropertyPanel
+        // so now this widget is drawn by the gray color for the visibility.
+        Gtk.CssProvider css_provider = new Gtk.CssProvider();
+        try {
+            css_provider.load_from_data(
+                    "GtkEventBox { background-color: gray }", -1);
+        } catch (GLib.Error error) {
+            warning("Parse error in Handle: %s", error.message);
+        }
+        Gtk.StyleContext context = get_style_context();
+        context.add_provider(css_provider,
+                             Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION);
     }
 
     public override void realize() {