build: Remove remaining artefacts from NowPlaying.X11 extension
Port the MeeGo extension to Gtk# 3

This is just a straight port to get the extension to build again.

There are some visual glitches when running on a standard GNOME 3
system, and various other issues. I'll address these issues in later
commits, along with some more structural changes.
Rename the MeeGo extension to MediaPanel and drop MeeGo integration

Rename everything from MeeGo to MediaPanel, with the appropriate
adjustments. All the MeeGo integration is also removed, so we can build
this extension by default. But the extension is not active by default.

The general idea for the MediaPanel extension is to provide a simple UI
to quickly browse and play your media. A bit like the MiniMode, but
less minimal. It is not specifically targetted  for netbooks, but it
will probably be more interesting for smaller screens, and maybe touch
devices ?
Remove all references to Hyena.PlatformDetection.IsMeeGo

PlatformDetection.IsMeeGo is only true if the file /etc/meego-release
exists, and it is only used for UI customisation that where specific to
MeeGo, and not useful for the MediaPanel UI.
MediaPanelService: Allow Banshee to quit when using the MediaPanel
MediaPanelContents: Improve layout and appearance

Remove the header, it was onyl useful when running as a MeeGo panel.

Don't use a custom color gradient for the background, just use the
default background.

Use Gtk.Grid instead of Gtk.Table, which is already deprecated.
MediaPanel.HeaderBox: Fix coordinates calculations for drawing

The Cairo.Context we now get comes from the OnDrawn event, so it is set
up for widget coordinates, and not windows coordinates as before. So we
need to update the various coordinates calculations to take that into
account.
MediaPanelTheme: Fix widget name prefix
Remove last remaining mentions of MeeGo