Change meta plugin activation condition

Current behavior:

1. A meta plugin activates, for each graphModel, that has a matching annotation.
2. If a MGL multiple graphModels have the same annotation, the meta plugin gets activates/executed multiple times.
3. If a MGL has no graphModels, then meta plugins cannot be activated.

Proposed behavior:

1. A meta plugin activates once, if any element in any MGL has a matching annotation.
2. No multiple activations of the same meta plugin. The plugin is activated/executed once per project/CPD.
3. Ability to activate a meta plugin without the need of a graphModel.

Closes: #315 (closed)