Fix snap build for Focal + update README
Fixes the snap build pipeline:
- Update snap base to core20 and build images to 20.04
- Fix "desktop-launch" command does not exist
- Update qml ubuntu modules to lomiri equivalents on Focal
- Update ubuntu toolkit to lomiri
- Update ubports repo to Focal
Also fixes "wayland-egl" plugin missing error by adding qtwayland5. Plus small README updates.
Tested locally on my Arch sytem using snapcraft and lxd build provider.