More prominent staged/unstaged via gutter

[DragonDev1906]

In the git diff view (where you can see all changes and stage/unstage them) I find it really hard to see which changes are staged and which aren't. The only difference is that the background gets slightly darker/transparent if staged (which I don't find intuitive either).

Suggested changes to the gutter:

• Yellow/Green/Red: Not staged
• Blue: Staged (same color as git show)
• (optional) Gray: Explicitly marked as not-staged (for example because it should be in a later commit, see below)

This should would also make it possible to see which changes are staged while in a normal code editing pane. At the moment the stage/unstage shortcuts work there but there is absolutely no visual indication.

Why a third state (seen, marked as staged/unstaged)?

Although it doesn't help with readability in regards to what is staged and what isn't, it does help seeing which changes you've seen and marked as not being part of the commit and makes it easier to jump to new changes (editor::GoToUnseenHunk and editor::GoToPreviousUnseenHunk).

I see this seen vs unseen as a nice-to-have, but marking the staged changes with blue in the gutter would help A LOT.