Maneuverplanner modularization

Maneuvers are now implemented in separate classes.
Each class draws its GUI and has a method to compute the node.
Parameters are inside the Operation object and are not shared anymore.
Implement a (new) combobox utility

Switch maneuver planner selection to the new combobox
Fix steam directory under Linux
Merge pull request #403 from xytovl/modularManeuverPlanner

Modular maneuver planner