Stashing individual hunks or lines

[jugglerchris]

Is your feature request related to a problem? Please describe.
I sometimes want to stash only parts of files - for example some dirty debug code when mixed with "real" changes, when I might want to recover the debug code after testing the change without.

Describe the solution you'd like
When selecting changes to stash, allow selecting individual parts of files to stash rather than stash everything, like git stash push --patch.

Describe alternatives you've considered
Not using gitui for this. :-)

[extrawurst]

depends on #59 (the way we decide how to do the UX of that)
depends on #106

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[extrawurst]

#1116 is a continuation. Please help me get some traction on the upstream limitation that keeps us from implementing it