Add support for one palette per frame in animations

[Thomas Bernard]

I have found some animations that have one palette per frame instead of a global palette.
This is supported in GIF and ILBM formats

it could be an option accessible only in Animation mode
Some features would need to be added in order to copy the palette from one frame to the others, etc.

see iff/anims/DefenderOfTheCrown/Z.abr in pic-samples repo

[PulkoMandy]

This will require some work for the undo/redo of the palette, I think. Everything else should not create too much problems.

[yrizoud@…]

Oh yes there are lots of (functional) issues.
Say you have palette A for frame 1 to 10, and palette B for frame 11 to 20. If user swaps frames 10 and 11, you have to add 2 new palettes:
A is for 1-9
B is for 10
copy of A is for 11
copy of B is for 12-20

Queue the hairpulling of user who tries to keep his palettes of frames 1-9 and 11 synchronized.

[PulkoMandy]

Move open tickets to 2.7 milestone