WIP: Initial GTK4 port

Pretty much non-functional GTK4 port, done by disabling anything that did not work out of the box.

It should run however and the songsview does play.

Note 17 February 2020

Currently needs an older checkout of GTK4 to work, because of changes to templating and it not being supported in PyGObject yet.

Note 29 April 2020

Templating support is working with pygobject!145 (merged). This is rebased and working again to some extent.