Functionalities:
- Mouse, touch and pinch input handling for MarbleQuickItem (keyboard
was enabled earlier)

Other changes:
- Refactored input handling to reuse code, broke into smaller pieces
(functions)
- Extended the example (marbleQuick2)

Known issues:
- Some refactoring still should be done around input classes
- Inertial rotation is disabled by default in MarbleQuickItem because
of some issues (mostly touch-related). However, issues are rather
minor so feel free to enable it back.

REVIEW: 118495