Fix cmd+Q invalidating the layout somehow

On Mac OSX Qt Applications (with Frameworks packaged) receive the
CloseEvent twice. (See https://bugreports.qt.io/browse/QTBUG-43344)
This triggers a bug where Quassels Main-Window stays hidden and all
Layout states get reset on next launch.
Workaround by checking if event already received...

Resolves GH-230.

(cherry picked from commit 89175f57858e885d23dac1401f6f14db20ba9002)