Fix build against Weston 1.3
Make setting XWayland windows title work
Fix memory leak in signal.h
Fix warning for missing initializer for member weston_mode::link
weston: Don't hide inactive transient windows
weston: Make sure popups don't get shown or hidden too fast

An annoying bug related to QQuickWindow hangs the client process when
showing or hiding windows too fast.
weston: Release popup grab when the surface is unmapped

Popup windows are hidden when dismissed so we implicitely release
the grab (this avoids a new request to do it explicitely).