multitouch support
MouseArea doesn't support multitouch. On android, when you holding one button, you can still click other buttons. But if using MouseArea based keypad, one button blocks other buttons from receiving key press. This MR replaces MouseArea with Qt QuickControls 2 AbstractButton
Note: I skipped src/qml/Call/AnswerSwipe.qml because it will need more than just substitute the MouseArea with AbstractButton, though it's better to replace it as MouseArea conflicts with each other if on same page.