This fixes a bug in the old implementation where the application could
crash after a second shell window was created and destroyed, because a
signal handler with the destroyed shell window as the closure was left
connected.
But moreover this simplifies the implementation by using a property
binding plus transform function instead of juggling signal handlers,
and also adds code comments where things get a little tricky.
Removed (now unused) functions:
e_shell_window_get_toolbar_new_prefer_item
e_shell_window_set_toolbar_new_prefer_item
