[Unison-hackers] Update and cleanup plan for the macOS UI

[Michael Roitzsch]

Hi Unison devs,

As I mentioned in my pull request for supporting macOS Mojave’s dark mode (https://github.com/bcpierce00/unison/pull/236), I would like to invest some work in the macOS UI. But before I begin, I want to clarify what changes I have in mind and discuss if you feel they are acceptable.

I do not want to change much in the way the UI works. I am quite used to it and I think it works well the way it is. I also do not want to upgrade the minimum deployment target beyond the current 10.6 unless necessary for some important feature. I would ask back here, if that becomes an issue. I also do not plan on introducing any new dependencies.

I would propose the following updates and cleanups:
* cleanup the build system, including the Xcode project (newer Xcode shows warnings there)
* cleanup the code formatting (I see some mixture between tabs and spaces)
* fix warnings (and deprecations, as far as possible within the 10.6 target)
* replace the hardcoded colors to be better suited for dark mode
* upgrade all artwork for dark mode and high DPI („Retina“) support
* enable Cmd-C (copy) to copy the path when a file is selected
* maybe add a contextual menu to provide quick access to some actions (diff, ignore, reveal in Finder)

Two questions I have:
* Would it be OK to rename the folder from uimac14 to just uimac? Since this is the one and only macOS UI, the extra 14 seems confusing.
* What do think about localization? Is it too much effort to maintain in the future? Otherwise, I might also consider a German localization.

If this proposal is welcome, I would work on this in the coming weeks. I don’t want to make any projections for when I will be finished, though. My progress will be visible in my fork on GitHub: https://github.com/mroi/unison (I will take the liberty of reordering commits or rebasing against upstream at any time, so do not expect fast-forward updates.)

Regards,
Michael

PS: Finally, I should thank you for this great tool. I heavily use Unison as part of my daily routine and it has easily helped me keep my sanity when working across multiple machines.