Icon-O-Matic: Generalize some classes

* Turns PathContainer, ShapeContainer, StyleContainer into Container.
* Makes Shape use Container for storing Tansformers instead of
reinventing the wheel yet again.
* Moves common logic from AddPathsCommand, AddStylesCommand,
AddShapesCommand, and AddTransformersCommand to AddCommand.
* Factors out common logic from two more commands similar to above.
This adds the MoveCommand and RemoveCommand classes.
* Cleans up some code style.

Change-Id: I59eca94a1c45eac608514f269f6e5f8628f93fab
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6640
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Icon-O-Matic: Save alpha of reference images

Change-Id: I821521c2f58052d70e9f27c7ffc9849095e540a0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6658
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

Icon-O-Matic: Add reference images

Fixes #2748

Also fixes a comment misstating a function's name and code style.

Change-Id: I609a1f1e100ded647818e70b428cedc48cf29036
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6604
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>