Metadata editor framework

[mih]

This brings a new "Metadata" context tab, and an editor widget for git-annex metadata (which can load/save from/to the repo).

Importantly, this is not done in a generic fashion using a CLI implementation, but GUI-native. Main reason is lack of ability of the author.

• Closes #280
• Closes #266

https://user-images.githubusercontent.com/136479/195285767-7ae675f5-51b9-40cb-8048-f4c67b162404.mp4

TODO

• [x] annex metadata field names (as opposed to values) have stricter requirements. Validate those too.
• [x] make fields loaded from an existing metadata record non-discardable. otherwise users will be confused that their records remain

[codecov-commenter]

Codecov Report

Base: 69.08% // Head: 63.87% // Decreases project coverage by -5.21% :warning:

Coverage data is based on head (25c203c) compared to base (c1d9773). Patch coverage: 23.01% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #315 +/- ## ========================================== - Coverage 69.08% 63.87% -5.22% ========================================== Files 43 47 +4 Lines 2824 3200 +376 ========================================== + Hits 1951 2044 +93 - Misses 873 1156 +283 ``` | [Impacted Files](https://codecov.io/gh/datalad/datalad-gooey/pull/315?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [datalad\_gooey/fsbrowser.py](https://codecov.io/gh/datalad/datalad-gooey/pull/315/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9nb29leS9mc2Jyb3dzZXIucHk=) | `39.20% <2.77%> (-0.80%)` | :arrow_down: | | [datalad\_gooey/annex\_metadata.py](https://codecov.io/gh/datalad/datalad-gooey/pull/315/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9nb29leS9hbm5leF9tZXRhZGF0YS5weQ==) | `16.73% <16.73%> (ø)` | | | [datalad\_gooey/app.py](https://codecov.io/gh/datalad/datalad-gooey/pull/315/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9nb29leS9hcHAucHk=) | `68.51% <18.18%> (-2.48%)` | :arrow_down: | | [datalad\_gooey/flowlayout.py](https://codecov.io/gh/datalad/datalad-gooey/pull/315/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9nb29leS9mbG93bGF5b3V0LnB5) | `25.00% <25.00%> (ø)` | | | [datalad\_gooey/metadata\_widget.py](https://codecov.io/gh/datalad/datalad-gooey/pull/315/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9nb29leS9tZXRhZGF0YV93aWRnZXQucHk=) | `58.82% <58.82%> (ø)` | | | [datalad\_gooey/metadata\_editor\_base.py](https://codecov.io/gh/datalad/datalad-gooey/pull/315/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9nb29leS9tZXRhZGF0YV9lZGl0b3JfYmFzZS5weQ==) | `92.85% <92.85%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad)

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[jsheunis]

Demo on a mac:

https://user-images.githubusercontent.com/10141237/195296133-63730d12-b90a-4b0d-bc13-114941fb1dc1.mov

(I thik i might have captured a bug in this video as well: delete metadata fields --> save --> metadata fields come back?)