Add select flag for motion and click actions

[Riateche]

Added select flag to Action::Motion and Action::Click to implement hotkeys with Shift modifier (e.g. Shift + ArrowRight to create or extend the selection) and fix hotkeys without Shift (e.g. ArrowRight previously didn't clear existing selection).

Fixed Previous and Next behavior when there is a selection. They now clear selection and put the cursor to the start and end of the selection, respectively.

Implemented select flag for examples.

[Riateche]

Rebased onto main. Please take a look.