Introduce a pan backend for touchscreen gestures

[leolost2605]

This allows exact finger tracking for touchscreen devices. Fixes #1272

Fixes #1801

In the long term with some coming mutter changes it will handle all gestures (touchpad as well) in order to remove the need of touchegg which currently only works because of xwayland.

This will also allow us to have much more precise gestures: e.g. swipe up from the bottom to reveal the dock one to one will be trivial to implement (PR incoming). It can also be used to do swipe in from corners to use hotcorners on touchscreen (another PR incoming :) ).

TODO:

• [x] Cleanup commits
• [x] Fix meta_wayland_pointer_set_focus: assertion 'meta_cursor_tracker_get_pointer_visible (cursor_tracker) || clutter_seat_is_unfocus_inhibited (clutter_seat) || surface == NULL' failed

For easier review see the individual commits and also their message for details about some changes. Rebase merge

[leolost2605]

This should be ready for testing/review now