Fix wrong behavior and crashes in fixture manager when deleting fixtures

Strange behavior : If a fixture is present in two or more fixture groups, and you delete, it, it will stay present in one group out of two. If you delete it again, crash ! (because it has been deleted in the document)

Another fun way to make qlcplus crash :
- Have a fixture present in two or more fixture groups
- Select everything in the fixture manager
- Delete
- crash !

This patch hopefully fixed that.
Feel free to comment / correct.