toast: Add custom-title property
Allow to change the title's GtkLabel with a custom widget.
I didn't add it to the demo because I'm not sure what example would make sense. Our use case is a custom label that allows us to insert a "pill" with an avatar and the name.
Further context: https://matrix.to/#/!hwiGbsdSTZIwSRfybq:matrix.org/$16435547458924rKpAg:gnome.org?via=matrix.org&via=gnome.org&via=privacytools.io
Fixes #339 (closed)