Gedit had that feature I believe. Next to the line ending button in the bottom toolbar, add the same for indent settings. Like this:
[ ] Spaces
[ ] 8
[ ] 2
To avoid duplicate work, I'm just dropping a note that I'm currently working on this. It mainly just needs some cleanup and bug fixes (and probably a code review) before submitting a pull request.
I now have a mostly-working version of this on my fork:
I don't consider it quite finished yet, but it's feature-complete.
The main changes are:
Anyway, there's still some cleanup to be done, mostly to do with the UI. For example, it isn't communicated right now which Editor-wide indentation settings are defaults rather than absolutes. But if people want to give this a spin and report bugs, that would be great. :-)
I now consider this feature "finished", pending any requested changes from Mezomish. I've submitted a pull-request here: