Selecting All slices and Delete won't delete all slices under a specific condition

[Gasparoken]

If an aseprite file has a slice that varies in size/position across frames, this slice will have "keyframes".

Current behavior:
The actions under Slice Tool:
All then Delete
will delete only the active "keyframe" (or the nearest "keyframe" at the left of the active frame) on a slice with multiple keyframes, i.e. this slice won't be deleted. On the other hand, all slices withouth "keyframes" will be correctly deleted.

Expected behavior:
All slices should be deleted.

Related: #2283 (comment)
Related #721

Aseprite and System version

• Aseprite version: 1.3.6
• System: all