Add support for Multi-touch

[SnoutBug]

This PR enables support for Multi-touch devices.
A Multi-touch device is similar to a touchpad, but it enables the use of multi touch gestures, by providing contact points on the virtual surface of the multi touch device to the user, which can then be moved individually.

The test function TestBasicMultiTouchGesture() should attempt to bring up the overview in GNOME or KDE using Wayland.

[bendahl]

Hey @SnoutBug,

first of all, thanks for this pull request. This is generally looking pretty good, but there is one remaining issue. Upon running the provided tests, the following error occurs:

--- FAIL: TestMultiTouchMoveToFailsOnClosedDevice (0.22s)
    multitouch_test.go:125: Expected error due to closed device, but no error was returned.

The error is reproducible locally. Looks like the following check needs to be fixed ('==' instead of '!='):

if err != nil {
		t.Fatalf("Expected error due to closed device, but no error was returned.")
}

Can you please fix this issue, so I can rerun the tests and merge your pull request?

Again, great work regarding the multitouch functionality. I'm excited to get this integrated!

Cheers
Ben

[SnoutBug]

Thanks @bendahl, I fixed that!

[bendahl]

Awesome! The changes have been integrated into the master branch! Thanks again for your efforts!