Implement touch drags

[ifreund]

River now has basic touch support as of #632, but touch drags are not yet implemented.

I took an incomplete stab at this here but it's pretty tricky and I don't have hardware to test on: https://github.com/riverwm/river/tree/touch-drags

Related: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3452