Assorted cleanups of `prefsdialog`

as i work towards customizing the network interfaces to monitor, i come across small things i'd like to clean up. i push the commits separately to make it more manageable.

towards the end of this set are 2 different ways of removing an ugly string switch in create_field_page():

• one uses templates: it cleans up the code but it should produce a very tiny bit larger binary.
• the other uses activate-on-single-click=true: it makes the whole column selection row in settings clickable on single click instead of double click, and thus the "toggled" signal handler and the ugly switch are no longer needed.

cherrypick whatever you want.

thanks!