GenericUI: only connect one PropertyChanged callback

GenericUI: only connect one PropertyChanged callback

The code connected the callback to the PropertyChanged signal from the
plugin once per filepath control created. Should the plugin have several
files to open, this would be at best wasteful and at worst racy.

Connect the callback a single time, since the same callback handles all
property updates that we're interested in. Also rename the methods,
members and typedefs so that it's clear what the code is trying to do.