iOS: let qmake generate default LaunchScreen.xib

iOS8 will check if the app has a LaunchScreen.xib to determine
if it supports iPhone6/6+ (scale factor and resolution). So
we follow the same pattern as we do with the launch image for
iPhone5, and generate a default LaunchScreen.xib.

The xib file in this patch is a copy of a default file
generated by a native Xcode project (with quotes escaped), but
with the text label set to be $$TARGET.

Change-Id: I163ab48b6f4edea4cc1f6840a1f3d8b3cc0326db
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>