Migrate to Ubuntu Touch 20.04 Focal

This MR adapts Dekko to Focal.

• Update framework, apparmor policy and desktop file (X-Ubuntu-* to X-Lomiri-*)
• Update to recent enough Clickable (or just latest) and update CI config (use 20.04 CI image)
• Update Python (3.5 to 3.8)
• Bump version
• Test on desktop mode (at least the GUI)
• Replace upstart service by systemd service
• Replace Ubuntu QML types by Lomiri ones
• Fix dekkod permission issues
• Fix dark mode (solved on system side)
• Fix Dekko+Lomiri crash (ubports/development/core/lomiri#45 (closed))
• Merge updated submodules (qml-richtext-editor!1 (merged), notify!1 (merged)) and then update .gitmodules
• Fix notifications
• Remove packaging of pyotherside
• Thoroughly test all features
• Update snap packaging (needs to be addressed separately)
• Switch to DBus for systemd interaction (just nice-to-have)
• Fix cross-compiling for whom does not have English set as system language (works in CI and for most people)