Add mouseup to the mouse observer

[pjasiun]

Contextual toolbar sample needed mouseup event, it might be useful for others too. It should be added to the mouse observer.

https://github.com/ckeditor/ckeditor5-ui-default/blob/4e1408fe3fd986306b79ad6ed428663e3264fc56/tests/manual/contextualtoolbar/contextualtoolbar.js#L96-L104

[jodator]

I'm gonna need this in ckeditor/ckeditor5-table#63. So I'm taking this to not forget that this one have an issue already.

[CKEditorBot]

There has been no activity on this issue for the past year. We've marked it as stale and will close it in 30 days. We understand it may be relevant, so if you're interested in the solution, leave a comment or reaction under this issue.

[CKEditorBot]

There has been no activity on this issue for the past year. We've marked it as stale and will close it in 30 days. We understand it may still be relevant, so if you're interested in the solution, leave a comment or reaction under this issue.

[CKEditorBot]

We've closed your issue due to inactivity over the last year. We understand that the issue may still be relevant. If so, feel free to open a new one (and link this issue to it).