Felipe Borges authored Feb 12, 2018 and Felipe Borges committed Feb 16, 2018

Window groups (Gtk.WindowGroup) were introduced in order to allow
multiple windows running simultaneously and to prevent focus/input
conflicts between them and their respective dialogs such as wizard
and properties.

By including the toplevel window in its own window group within its
dialogs we can guarantee that the "modal" behavior is respected for
each group. e.g., an open dialog prevents any interaction with its
toplevel window but doesn't not interfere with other window groups.

Fixes #80