Draft: Add Button to install software

Successor of !119 (closed), though I am not sure if I should merge this.

I only got it to run with gnome-software ... though it seems I broke some dependency while experimenting with other software installers / gnome-software does not start anymore for me, nor do any others 🙁

In theory, afaik the packagekit API should as well be supported by e.g. apper (and others?) ... though I have no idea how to get it to work.

@lastonestanding, @Tamaranch : Would be great if you could give it a spin / let me know if you think we should add it at all.