AnimationPlayer: show values in the Inspector also when selecting different key types

[golddotasksquestions]

Describe the project you are working on: Projects with lots of animation and keys of similar as well as different types.

Describe the problem or limitation you are having in your project:
When multi-selecting more than one key, the idea is to save effort by doing an operation once for all keys. Unlike 3.0.6, in 3.1, multi-selecting keys of the same kind to edit their value is not possible. Thanks to this PR it's back in 3.2.
If however in 3.2. beta4 there is even just one key among the selected ones that is not of the same type, no values other than the timestamp position are made available in the inspector.

Describe how this feature / enhancement will help you overcome this problem or limitation:
If the values would show up in the Inspector, we could edit all values of keys of equal type simultaneously and would not have to sift through the masses of blue points find the ones that are of the same type, edit the values, and start the whole process again. It would be a tremendous time save and usability improvement.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:

Describe implementation detail for your proposal (in code), if possible:
Wish I could

If this enhancement will not be used often, can it be worked around with a few lines of script?:
Don't think it can.

Is there a reason why this should be core and not an add-on in the asset library?:
It's a necessary usability improvement allowing efficient AnimationPlayer use.

[YuriSizov]

This feature is approved and PRs are welcome! One note from the review meeting is that in the clip in the OP easing doesn't seem to be considered a shared property, but that's like a bug. So for the example in the OP, both time and easing should be shared, and the rest of the sections can be just for values.

On a personal note, probably a bit more distinction for the two groups, shared and individual, is needed. Maybe with the use of sections, and having individual properties as groups.