With this PR the following 5 command modes are automatically stored in the preferences:
* "ContinueMode"
* "RelativeMode"
* "GlobalMode"
* "fillmode" (existing preference, removed from the Preferences Editor)
* "SubelementMode"
Additionally:
Three commands (Draft_Ellipse, Draft_Rectangle and Draft_Stretch) call `setRelative` after the 1st point has been specified and would permanently switch on relativeMode. After this PR this switch is temporary.