Clear clipboard when diagram items are copied

[amolenaar]

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?

Copy /paste behavior is not intuitive when. Also it conflicts with copy/paste in the editor pane.
E.g.

• Create a class item on the diagram
• Copy it
• go to the name field in the editor. Select all (ctrl-A), cut (ctrl-X) and paste (ctrl-V).
• An item is pasted in the diagram. No change in text in the editor pane.

Issue Number: #274

What is the new behavior?

Gaphor interacts with the system clipboard. Clearing it's diagram item copy buffer when something else is copied.

For now and empty string is put on the system clipboard. This should change to an image of the copied items at some point.

Does this PR introduce a breaking change?

• Yes
• No

[sourcery-ai]

Sourcery Code Quality Report (beta)

❌  Merging this PR will decrease code quality in the affected files by 0.12 out of 10.

Quality metrics	Before	After	Change
Complexity	1.50	1.64	0.14 🔴
Method Length	32.09	36.71	4.62 🔴
Quality	9.06	8.94	-0.12 🔴

Other metrics	Before	After	Change
Lines	94	111	17

Changed files	Quality Before	Quality After	Quality Change
gaphor/services/copyservice.py	9.06	8.94	-0.12 🔴

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 via email or our Gitter!

[danyeaw]

Nice update!