Preparing 2.3.1 release;
Merge pull request #398 from MuMech/dev

MechJeb 2.3.1 release
Removed revision checking from CompatibilityChecker.
Merge pull request #399 from MuMech/dev

Removed revision checking from CompatibilityChecker, In case we get a KSP 0.24.3 >.>
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
Revert "Merge pull request #403 from xytovl/modularManeuverPlanner"

This reverts commit 28279d456a5e9351b67b8aee335285a2f52496a8, reversing
changes made to f10b429fdb0446594107c525228337fa734950fb.