Fixed GUI s.t. it doesnt enable collision detection during initialization

Removed previous workaround that prevented that behaviour.
Combined the enabling and disabling of collisions with the selection of the collision detection method.
Merge pull request #48 from kstrel/uifix-collisions2

Fixed GUI s.t. it doesnt enable collision detection during initialization

Previously we had a boolean variable in the Simulation class that enabled and disabled collision detection (even though we actually have empty detector classes for that). That also meant that setting a non-empy detector algorithm in the Settings class (for the non-gui version for example) wouldnt actually enable collisions. The boolean in the Simulation class would have had to be changed too.

The boolean was a workaround because without it the GUI always enabled collision detection during initialization. I have fixed it now. The problem was that detectorChange() and algorithmCollisionChange() from Particle2DPanel were called during initialization and these two methods could only set non-empty detector classes.

Furthermore I have combined the enabling and disabling of collisions with the selection of the collision detection method into one dropdown menu.