Add Settings entry to File menu

This automagically creates a (localized?) Preferences entry on OSX as well
Fix #66 Display Capture for HiDPI displays

kCGDisplayStreamSourceRect takes points rather than pixels
Track properties window lifetime via QPointer

Previously the properties window would sometimes not receive
a closeEvent, leaving a dangling pointer in OBSBasicMain (and resulting
in a crash when trying to open a new properties window or closing
the application)
Allow hiding the cursor for Display Capture