2.1.1

1. Fixed crash on window destruction. Thanks to @JulienMaille !
2. Removed the setVisible call for both QtWidgets and QtQuick implementations, fixed window geometry issue for some applications. Thanks to @JulienMaille !
3. All the example projects gained the ability to record and restore the preivous window geometry. Thanks to @JulienMaille !
4. Simplified QtQuick implementation. The three QtQuick system buttons now shared one same code base.
5. Corrected some color calculation mistakes of the system buttons.
6. Removed some unused bundled resources.
7. Added function to retrieve the runtime version of FramelessHelper: int FramelessHelper::Core::version().
8. Other internal refactorings and tweaks.