clean-up gesture tracking code in libview and improve touchscreen support

This MR

• moves the selection and annotation gestures to a drag gesture,
• splits them into their own drag gestures,
• and blocks them for touchscreens,
• uses claiming gestures, to cancel other gestures, (Note: done, except comment below)
• replaces custom velocity calculations with gesture swipe,
• makes the page change swipe gesture more versatile in allowing more directions, and
• introduces a long press gesture for the context menu.

Not included are, for now, a way to move annotations on touchscreens, selection text besides the double/triple tap gestures. For that, I would need to ask the Gtk.ScrolledWindow to hold its scrolling gestures. I follow up on this later.