screen: Split workspace initialization from meta_screen_new()

Since commit 8b2b6524, we assume that the compositor always
exists. Alas, the assumption is wrong - the compositor is currently
initialized after the screen, but meta_screen_new() itself may
call a compositor function if initialization involves a workspace
switch (which will happen when meta_workspace_activate() is called
more than once and for different workspaces - or in other words,
when _NET_CURRENT_DESKTOP is set and not 0).
So carefully split out the offending bits and only call them after
the compositor has been initialized.

https://bugzilla.gnome.org/show_bug.cgi?id=731332