Merge together the functionality of simulation and modelling

Instead of having a simulation button, the play button (in the bottom left corner) should always start a simulation. When the user clicks somewhere in the timeline, the simulation is reset and the state of the Grid is shown without taking the past into account.

The "forward one step" button should simulate only one step (including calling the algorithm if one is selected).

The "go to beginning button" should reset the simulation and place the time in the original position.

The "backwards one step" button should be removed as we do not plan to be able to go back in time to see what happened in the previous simulation step (this would require saving the full state of the network at every point in time. Maybe in the future wink)

#17 Merge together the functionality of simulation and modelling