Revamp InputLine

* Use {Q|K}TextEdit rather than QLineEdit on all platforms - we need this for fancy stuff to come
* Replaced hacky hardcoded height calculation by proper QStyle-based information, should finally
  fix the jumpy input line everywhere, the oversized widget in Oxygen, and other issues
* Use sizeHint() rather than setting a maximumHeight() to make the input line work in layouts
* Cleanups