properties: Derive from Gtk.Notebook

Now that we are going more from containership to inheritance in other
widget classes to be able to make use of UI templates, lets do the same
for Properties class for consistency.

Moving this class' UI setup to .ui won't be so easy as most of the
contents are dynamically created.
properties: Derive PageWidget from Gtk.Grid

Similar in spirit to previous patch.
editable-entry: Move UI setup to .ui file
empty-boxes: Move UI setup to .ui file
wizard-source: Make URL entry visible
notificationbar: Seperate classes for Notifications

Put notifications themselves into seperate classes/modules.
fixup! editable-entry: Move UI setup to .ui file
notification: Move UI setup to .ui file
auth-notification: Move UI setup to .ui file