Haiku: Adapt infrastructure to compile qtbase under Haiku OS
Haiku: Initial version of QPA plugin for Haiku

The plugin at this stage supports the following features:
   - QtWidgets: Shows all QtWidgets on screen
   - mouse and keyboard interaction (including scroll wheel)
   - window handling (minimize/maximize), fullscreen still missing
   - clipboard handling
   - cursor handling
   - basic support of QScreen