ui: Add animation support for the edited/moved marker.

[PIG208]

This is a sequel to #762 that animates the swipe gesture.

[PIG208]

Comparisons:

Collapsed	Expanded

[PIG208]

The appearances of the markers can be checked in the comparisons table above.

Incorporating @chrisbobbe 's comment, we animate the marker from the start state to the end state as follows:

	margin left	margin right	paddings (horizontal symmetric)	distance between marker text and icon
Collapsed	5px	3px	0px	0px
Expanded	13px	3px	3px	5px

where the paddings are the distances "between the left edge of the rounded-rectangle and 'Edited'" and "between the icon and the right edge of the rounded-rectangle", the margins are the distance between the rectangle to the left of the screen and to the message content, respectively.

[PIG208]

Rebased since #762 has been merged. We will need to get back to this later to investigate alternatives to LayoutBuilder.

[chrisbobbe]

My review is next, I think, right? I've just added "maintainer review" so I don't lose track of it.

[PIG208]

Yeah, this is not ready for review yet, but I will let you know once it is.

[gnprice]

Marked as draft because that makes it easier to remember (particularly when looking at a list of PRs) which PRs have become ready for review 🙂