This PR makes Ulauncher dbus activatable, so you can launch it with gapplication launch io.ulauncher.Ulauncher, checking if the app id is already running at first, otherwise using the dbus service file to launch it.

The benefit is:

1. When starting Ulauncher via the Desktop entry (using menu via another launcher or gapplication launch io.ulauncher.Ulauncher it now starts the systemd service for us instead of just running the app. This way ulauncher will restart if it crashes.
2. This partly removes the need for ulauncher-toggle. One of two reasons to use ulauncher-toggle instead of ulauncher currently is that it's faster, but gapplication launch io.ulauncher.Ulauncher is actually even faster than ulauncher-toggle

The other reason to use ulauncher-toggle is to pass a query, but we should be able to do that with gapplication action io.ulauncher.Ulauncher set-query "asdf" or possibly just gapplication launch io.ulauncher.Ulauncher "asdf", although the latter case will expect a "asdf" to be a file )

Checklist

• Verify that the test command ./ul test is passing (the CI server will check this if you don't)
• Update the documentation according to your changes (when applicable)
• Write unit tests for your changes (when applicable)