[MetaScreen] Keep num_workspaces key in sync with the actual workspace number

Changing the number of workspaces via an external pager relies on the gconf
key; if a plugin adds or removes workspaces on the fly, we can get into a
situation when the stale number stored by the preferences matches the new
number requested by the pager, in which case the pager request becomes a nop.

This commit ensures that when the meta_screen_append_new_workspace() or
meta_screen_remove_workspace() functions are called, the stored value is
updated accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=613127