A lot of our modal dialogs share a similar structure:
[Icon] Some title
Maybe a subtitle
And sometimes even a body for stuff like
longer descriptions.
A dedicated widget with a common style will allow us to significantly
reduce duplication of both code and CSS.
https://bugzilla.gnome.org/show_bug.cgi?id=784985
This is the basic dialog actor implementation, which will allow us to
use the same implementation on the session-global modal dialogs. The
ModalDialog class now uses it underneath, and so do all users of it.
https://bugzilla.gnome.org/show_bug.cgi?id=762083