[leolost2605]

OS 9 doesn't package touchegg so we have to implement it ourselves. This also allows some improvements (most notably one to one finger tracking of touch screen gestures: #1272 )
See the individual commits for an easier review:

• The first four commits are preparation:
  • we want to allow a backend to send gesture progress in different units. E.g. touchpad wants to send in percent whereas touchscreen wants to send logical pixels to make sure a swipe of an actor follows exactly the finger
  • add some more infrastructure related to getting the distance a gesture is expected to move an actor (also needed for one to one finger tracking of a touchscreen gesture)
  • some misc stuff (see commit messages)
• then actually introduce the backend
• and enable it in swipetrigger and globaltrigger

Fixes #1272
Fixes #1801

The new TouchScreenSwipeBackend is only available when compiled against mutter 49 or higher (because there the new pangesture was introduced), older versions have to rely on touchegg.

Changes in behavior compared to touchegg are:

• When swiping etc. the actor tracks the finger 1:1
• In the multitasking view you can use one finger to switch workspaces

In this PR it's currently not yet used for the edge swipe gesture but I already have a working follow up for that (#2950)

[lenemter]

Where did this value come from?

[leolost2605]

Nowhere :) It's currently not used because zoom uses pinch and that's not implemented yet and even when we implement it we probably don't use it because with pinch we want to send a percentage (based on current distance between fingers and initial distance between fingers).
I kind of forgot I added that here 😅 I changed get_distance to be a virtual method and added documentation that get_distance only applies to swipe gestures