Allow sharing multiple resource trees/reference points in Saros/I

[tobous]

Saros/I currently only supports sharing a single resource tree/reference point.

The logic should be expanded to support sharing multiple reference points and adding reference points to an already running session.

After this functionality has been added, the base sharing logic has to be tested/reevaluated to ensure that sharing multiple reference points does not add any unintended side-effects/unwanted behaviors.

The following is a (non-exhaustive) list of steps that are required for this to happen:

• Add a wizard to select multiple reference points when starting a resource negotiation.
  • Add a menu option to display this wizard to the contact context menu.
• Expand the current logic when creating and receiving a resource negotiation to ensure that no nested reference points are created.
  • Don't allow the creation of a resource negotiation for base resources that are already shared as reference points or are children or parents of an already shared reference point or another reference point of the resource negotiation.
  • When receiving a resource negotiation, don't allow the user to choose a local representation that is already shared as a reference point or is a child or a parent of an already shared reference point or another local representation.
• Adjust the internal project handling logic (see SharedIDEContext.java). The reference points could be spread across multiple projects.
  • Replace usage of SharedIDEContext.getProject() with the IntellijReferencePoint.getProject() where possible. Maybe this can completely replace SharedIDEContext.getProject(), leading to it only being used for the event handlers.
  • Ensure that project event handlers are still started for all projects containing shared reference points.