Introduce new method areListenersPaused() to interface SelectionModel

[stefanhahmann]

This PR introduces a new method areListenersPaused() to the interface SelectionModel

• The method areListenersPaused() is used in BranchGraphSelectionAdapter to find out, if listeners have been paused outside of this method and only in case they are not perform
  • pause listeners
  • select the branch vertex
  • resume listeners
• In case they are already paused, then they are neither paused again nor are they resumed after selecting the branch vertex
• Without this change a selection of many branch spots at the same time caused that resumeListeners() was called with high frequency resulting in low performance, when selecting many branch spots at the same time

Resolves #322

[stefanhahmann]

Great finding and solution. Would you care to add some javadocs to the 3 listener methods in the SelectionListener interface?

Good idea. I tried my best to describe these 3 methods. Hopefully their function will be clearer with this in the future.