Add option to switch cameras

[igordsm]

This PR closes #41 and closes #59 . It adds a dropdown menu with possible cameras and switches to newly plugged ones on the fly. If more than one camera is present the last one provided by Gst.DeviceMonitor is used.

[danirabbit]

Hey thank you for working on this feature. This looks really great! I left a few inline comments to clean things up a bit.

One big thing I noticed is that it seems you included a workaround for menuitem labels that shouldn't be necessary with the latest revision of the stylesheet in Odin. So that should save quite a lot of code to not apply this

I think one thing to consider is that we should probably only show the menu button if there is actually more than 1 camera available. It would probably look nice to put this in a Gtk.Revealer with a slide transition

[igordsm]

@danrabbit @cassidyjames Thank you for the kind reviews. I'll work on them on the next days. After cleaning up I'll try implementing your suggestion regarding revealing the menu only when more than one camera exists.

[igordsm]

@danrabbit I added some animations with the Gtk.Revealer. The revealing animation is not smooth at all, but I'm running from an USB drive so I might be having some performance issues. It looks like this:

[igordsm]

I have removed the commented section and feel this should be ready for another review.

[danirabbit]

@igordsm can you resolve conflicts between this branch and master?

[igordsm]

@danrabbit Done.

[cassidyjames]

UX-wise this is super close for me! Would it be possible to add a RadioButton to the start of the menu items to show which camera is chosen?

[igordsm]

@cassidyjames Done.

[cassidyjames]

This works great for me. Awesome work!

Addressed