General code cleanup

- Fixed some clazy warnings (don't call first/last() on a temp. container)
- Remove QCoreApplication::flush() calls since it's deprecated upstream,
  those calls do nothing anyway (empty implementations in both the glib and
  unix code paths upstream)
- More range for
- Port to QGuiApplication::primaryScreen()
- More PMF signal/slot syntax
- Don't flush an iostream with every iteration through a for loop, just once
  after the loop should be enough