Add second panel for graphical display of phase space

The additional panel is especially useful for the "Two stream" initial condition. By default, it is collapsed, one can move the divider with the mouse.

Also, the class Particle2DPanel has been split into:
- ui.SimulationAnimation: contains the timer and logic of changing settings
- ui.Particle2DPanel: contains only the graphics output

The ui.PhaseSpacePanel is new and contains the graphical output of the phase space (position vs. velocity in one dimension).