MediaPanel: Allow switching between MediaPanel and main UI

The MediaPanel extension does not display itself systematically
anomore, only if Banshee is started through the MediaPanel client. It
now adds an item in the View menu to allow switching to it after
starting Banshee normally.
MediaPanel: Make the extension enabled by default

Currently, if the MediaPanel extension is disabled, running the
MediaPanel client will just silently fallback to the main UI. To avoid
this, the extension is now enabled by default, until we find some better
way to do this.

Furthermore, the extension is intentionaly not put in a category, so it
won't appear in the list of extensions in the preferences dialog, thus
can't be disabled.
MediaPanel: Add banshee-media-panel.desktop file