Add hover mode to the translatron

Similar to fixed vertical speed in the translatron. Only tries to fix altitude and points the nose upwards.

Mechjeb uses the engine to ascent/descent to a preset altitude. The player can maneuver the ship around like a helicopter, just tilt to move it in a certain direction. Usefull for moving short distances across surfaces and precision landings.

Adding a "Stop horizontal motion" button or toggle like the one in the vertical speed option would be a bonus.