ui/cocoa: release mouse when user switches away from QEMU window

ui/cocoa: release mouse when user switches away from QEMU window

This resolves an issue where using command-tab to switch between QEMU
and other windows on the host can leave the mouse pointer visible.

By releasing the mouse when the user switches away, the user must left
click on the QEMU window when switching back in order to hide the
pointer and return control to the guest.

This appraoch ensures that the calls to NSCursor hide and unhide are
always balanced and thus work correctly when invoked.

Signed-off-by: Carwyn Ellis <carwynellis@gmail.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>