Add ::first-frame signal to MetaWindowActor

This signal is emitted the first time a frame of contents of the
window is completed by the application and has been drawn on the
screen. This is meant to be used for performance measurement of
application startup.

https://bugzilla.gnome.org/show_bug.cgi?id=732343
meta_accelerator_parse(): handle keysyms without the XF86 prefix

The GDK and hence GNOME standard is that keys that begin with XF86 according to
libxkbcommon not prefixed with XF86, though gdk_keyval_from_name() strips XF86
if provided. If libxkbcommon doesn't recognize the accelerator name without
XF86, try again adding XF86 to the start.

This restores compatibility with gnome-settings-daemon, schemas, and existing
user configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=727993