Implements feature suggested in https://developer.blender.org/T52226.
This adds checkbox to offset new UVs by 1 on U axes.
This seems too limiting, (what if you're using mirror on U axis?)
Wouldn't it be better to have UV mirror offset number buttons?
Now there is additional values for offsets on both axes.
In the UI those properties should be split into two columns.
The first three images is what I had in mind
UI: properties split into two columns
@Campbell Barton (campbellbarton) do you have anything to add?