uisupport: Provide helpers for dealing with widget changes

In many places, we want to connect to a widget's changed signal,
i.e. a signal that is emitted whenever the widget changes. This
requires a great deal of boilerplate especially in settingspages,
since the signal to listen to varies between widget types.

Provide helpers that automate most of this by matching the given
widget against a static list of supported widgets' changed signals,
and connecting automatically. A sprinkle of template magic makes
it very easy to support a bunch of different widget types in a generic
way.

Make use of this feature to get rid of the remaining old-style
connects in SettingsPage. Also migrate a couple of specific
settingspages as proof-of-concept and API validation; others
can follow over time.