* Determine focused window when processing synthetic events

When a client is passively grabbing keybindings that it does not need,
it sends them up for other clients to process.

Often in this situation, the event contains the wrong window (either
root, for global keybindings, or the original client itself). This means
that Marco will attempt to process the event for the wrong window.

This is not an issue for global keybindings within Marco, as the focused
window does not matter. However, for shortcuts that operate directly on
specific windows, the event gets lost.

This change addresses this by determining what the currently-focused
window is, regardless of which client forwarded the event.

* Determine window under pointer when processing synthetic events

When a client is passively grabbing mouse clicks that it does not need,
it sends them up for other clients to process.

Often in this situation, the event contains the wrong window (either
root, for global keybindings, or the original client itself). This means
that Marco will attempt to process the event for the wrong window.

This change addresses this by determining what the current window under
the mouse pointer is, regardless of which client forwarded the event.

* Remove unused development data

_SVID_SOURCE has been deprecated in newer versions of glibc breaking -WError;
the recommended replacement of _DEFAULT_SOURCE is fairly new, so switch to
_XOPEN_SOURCE instead.

Based on mutter commits b159d6a5e17219557574defb6c3788d9b0c39757
and b159d6a5e17219557574defb6c3788d9b0c39757.

https://bugzilla.gnome.org/show_bug.cgi?id=763782

taken from:
https://git.gnome.org/browse/metacity/commit/?id=56bcf350fb599ec2caf68af4f0f92b1c3999027c

was my mistake in 9212dacb2bfd5985d8426bf0bdb88f2aca03444e :-/