screen: use stack tracker for initial window query

In meta_screen_manage_all_windows() we can use our own stack
tracker to get the list of windows - no need to query X again.

A copy is needed because the stack gets modified as part of the loop.
Specifically, meta_stack_tracker_get_stack() at this time returns the
predicted stack, and meta_window_new() performs a few operations
(e.g. framing) which cause immediate changes to the predicted stack.

https://bugzilla.gnome.org/show_bug.cgi?id=721345
meta_window_new: clean up error handling

The return code of XGetWindowAttributes() indicates whether an error
was encountered or not. There is no need to specifically check the error
trap.

The trap around XAddToSaveSet() was superfluous. We have a global error
trap to ignore any errors here, and there is no need to XSync() as GDK
will later ignore the error asynchronously if one is raised.

Also move common error exit path to an error label.

https://bugzilla.gnome.org/show_bug.cgi?id=721345
Reduce server grabs during window creation

Remove some obvious server grabs from the window creation codepath,
also ones that are taken at startup.

During startup, there is no need to grab: we install the event handlers
before querying for the already-existing windows, so there is no danger
that we will 'lose' some window. We might try to create a window twice
(if it comes back in the original query and then we get an event for it)
but the code is already protected against such conditions.

When windows are created later, we also do not need grabs, we just need
appropriate error checking as the window may be destroyed at any time
(or it may have already been destroyed).

The stack tracker is unaffected here - as it listens to CreateNotify and
DestroyNotify events and responds directly, the internal stack
representation will always be consistent even if the window goes away while
we are processing MapRequest or similar.

https://bugzilla.gnome.org/show_bug.cgi?id=721345