Add postmarketos-ui-sxmo

Add packages for Sxmo (http://sr.ht/mil/Sxmo) - an X based environment based on simple and suckless software.

Environment is optimized for the Pinephone but should have moderate compatibility with other devices (especially those that have volume up / volume down / power keys). Minimal testing has also been done on a Samsung Galaxy S4 Mini for basic graphical features. Cross-device compatibility should improve over time.