Port Properties to GtkBuilder

The UI freeze is in a few days, so it's time to merge the feature branch into master.

The work to be merged at this point has been reviewed in the following MRs:

!559 (merged) !561 (merged) !563 (merged) !570 (merged) !571 (merged) !572 (merged) !574 (merged) !576 (merged)

I'm going to reorder some commits to make the sequence more logical.

Main issue: #1207 (closed)