libicon: Notify Shape on transformer addition/removal

hrev57144 accidentally made Shape be notified only if the library was
compiled for Icon-O-Matic.
Also includes some miscellanious improvements.

Fixes #18510

Change-Id: Ie4ae7f9b8b1a5b39f87db2dbf79064fa875b644c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6705
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

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>