Remove special code path for fullscreen support on Mac

This was necessary for supporting full screen on 10.7 even while
keeping compatibility with 10.6. Since we no longer support 10.6,
we can remove the workaround.

Change-Id: I03e23e9203836ab6fb5a836fc06a525e44516d34
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Qbs: Use cache(...) instead of echoing system calls to write .qmake.cache

Qbs is nowadays disabled with Qt4 anyhow, so we can use the less
hacky Qt 5 methods

Change-Id: I9ce9391238b3597cc4b31e9f9f6647cec1a2496e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>