Gtk3: window-menu: Don't change selector style since it's already changed by libwnck

Based on gnome-panel commit: 60cb9c798ed25868c1c07b73fa08aabbb5892d2c
From: Carlos Garcia Campos <carlosgc@gnome.org>
GDK_KEY_* is also in Gtk+2.24
gir: Require g-i 0.9.5 and stop using deprecated --strip-prefix option
Gtk3: menu-bar: Remove duplicate code in panel_menu_bar_class_init

This is also called (correctly per [1]) in panel_menu_bar_init

Inspired by gnome-panel commit: 7230bad1c3bf2623d7fe2922422758ce56c95b16
Gtk3: panel: Explicitly check return value of cairo_surface_status()

Do not assume that CAIRO_STATUS_SUCCESS == 0. This clarifies the code

Taken from gnome-panel commit: d376b00a0a5856b3d5eb20a7dc1b39440d2f3938
Gtk3: panel: Draw the separator line where it should

The calculation wasn't done right. At least, I think so :-)
Also cleanup a bit some code in the file.

Based on gnome-panel commit: be822666a99680e5cd34c02ad2e35bea81f4a7d8
From: Vincent Untz <vuntz@gnome.org>
panel: Add panel_run_dialog_get_combo_text() helper

Instead of writing some long line again and again, use a small helper.

Taken from gnome-panel commit: 412ac2e4fa6a19c454b7046b9f57b9622a7b97d0
From: Vincent Untz <vuntz@gnome.org>
Gtk3: clock: Port to GtkStyleContext

Based on gnome-panel commit: c28ed51852a629884ce9ad8acc7924680c35854c
From: Germán Póo-Caamaño <gpoo@gnome.org>
Gtk3: panel, clock: Synchronize calculate_minimum_height() code

Also change the panel version to use the padding instead of the border.
We're interested in the padding here. I believe.

Taken from gnome-panel commit: 0b5d9e57451dff3a25b3fbcc8dec671cac469cb
From: Vincent Untz <vuntz@gnome.org>
Gtk3: panel: focus-line-width is not needed when drawing ButtonWidget

Based on gnome-panel commit: 8a942d16378f62015f302357e0e5b6f61b5713b3
From: Vincent Untz <vuntz@gnome.org>