WIP: Added measure as action that is reflected in history

After a discussion in #5698 I decided to try making a history of the measurements a thing.

This is my first GIMP code and first C code since uni, so I don't expect it to be any good.

So far, basic boilerplate to make measure tool events writeable to history and having getters to extract data out of - as opposed to simply printing the data into status field. I want measurements to be history events and to be stored in a list so that that list can be later used as "variables" in fields supporting math.

I am not sure what definition of done is - do you have some sort of test harness I need to extend, coverage, liters, all that?

"Build image" pipeline keeps failing for me with "Unable to locate package libraw19" error - not sure what's the deal there - i haven't changed any dependency related files.