window: don't process unrelated release events

When processing button events related to moving/resizing the window,
only the first button should be considered.

https://bugzilla.gnome.org/show_bug.cgi?id=704759
display: don't interrupt grab move/resize without modifiers

The part of code dealing with move/resize grab in display.c is only
responsible of this behavior when triggered with a modifier. So it
shouldn't stop the move/resize behavior triggered from a mouse event
without modifier on the title bar or sides of the window.

https://bugzilla.gnome.org/show_bug.cgi?id=704759