Fix ubuntu test build being broken by recent libhandy commit

This MR fixes the Ubuntu build test CI workflow being broken since libhandy@3628684b removed the files required for apt-get build-deps.
This is done by manually adding libgladeui-2-6 and valac as libhandy build deps and installing them together with the other build deps.
Some recent change also means that libsystemd-dev is required to build the system monitor when systemd support is enabled, so this MR also adds that as a dependency to install.

Closes #227 (closed)

Edit: Note that I have no idea why this makes it overall install more packages then with build-deps, when for all I know it should install less.
I have no idea how to fix that either tho, and it works like this so I think its fine.