main: More dead code removal

The View and Sound menus are now empty. The Remove right-click
menu for the playlist can also be removed (we can do the same
with the toolbar button in that widget).
grilo: Make TotemMainToolbar a GtkHeaderBar

So that we can get the right styling when using it as a
headerbar.
sidebar-test: Remove test plugin

The sidebar is gone from the UI...
main: Remove "show-menubar" hook

We don't have a menubar anymore.
main: Remove last bits of Properties in main
main: Remove all the sidebar handling code
properties: Remove mention of sidebar in description