Fix #408 where undo on diagram items breaks the model

[amolenaar]

diagram items end up in the ElementFactory, because they emited
ElementCreated and ElementDeleted events.

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Undo does no longer break the model. Instead DiagramItemCreated events are no longer inherited from ElementCreated events. This ensures both code paths do not intervene.

I added DiagramItemDeleted events, to match with the DiagramItemCreated events.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[sourcery-ai]

Sourcery Code Quality Report (beta)

❌  Merging this PR will decrease code quality in the affected files by 0.02 out of 10.

Quality metrics	Before	After	Change
Complexity	0.79	0.79	0.00
Method Length	24.83	25.65	0.82 🔴
Quality	9.32	9.30	-0.02 🔴

Other metrics	Before	After	Change
Lines	745	772	27

Changed files	Quality Before	Quality After	Quality Change
gaphor/core/modeling/diagram.py	9.13	9.14	0.01 🔵
gaphor/core/modeling/elementfactory.py	9.37	9.35	-0.02 🔴
gaphor/core/modeling/event.py	9.61	9.62	0.01 🔵
gaphor/core/modeling/presentation.py	9.53	9.49	-0.04 🔴
tests/test_undo.py	9.06	8.98	-0.08 🔴

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Overall	Recommendation
tests/test_undo.py	test_class_association_undo_redo	3	199.98	5.89	Split out functionality

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Let us know what you think of it by mentioning @sourcery-ai in a comment.

[danyeaw]

@amolenaar Nice job fixing this!