Um is a naming hangover from before this was part of g-c-c.
It wasn't correctly moved from the UM namespace when renamed in ec529e8. This hasn't caused a problem, it's just a little confusing.
ec529e8
Switch from GtkBuilder to using GtkTemplate. Rename .[ch] and .ui files to standard names. Rename widget IDs to be more readable. Drop widget IDs that are not used. Move code into the .ui file that can be. Connect signals in swapped form.