Add graphical solver

When launching MasterSim, use a graphical solver window instead of spawning a console solver (maybe make this optional).

The graphical solver shall run in a separate GUI thread with a worker thread for mastersim.
It should support "run completely" and "run step-by-step" mode. It can later include things like added stop marks, rewind (for FMU 2.0) and output of current exchange variables.