Modify inertia implementation for freelook and orbit

- freelook now uses position based inertia (holding a key for a
   specific time always rults in the same distance traveled independent
of inertia setting)
 - orbit inertia now is angle bases. (not transformation based) ->
   camera always takes the same path.
 - added setting for orbit inertia
 - added setting hints for freelook settings.

editor/plugins/spatial_editor_plugin.h

@@ -131,7 +131,7 @@ private:
 	float gizmo_scale;
 
 	bool freelook_active;
-	Vector3 freelook_velocity;
+	Vector3 freelook_target_position;
 
 	PanelContainer *info;
 	Label *info_label;
@@ -239,7 +239,7 @@ private:
 			distance = 4;
 			region_select = false;
 		}
-	} cursor;
+	} cursor, camera_cursor;
 
 	void scale_cursor_distance(real_t scale);