MAP: changed tracing to line

trace is now line. it needs to be fixed so that it's behind
the roomba-triangle and velocity-vector
MAP: bounding rectangle and visibility corrections

- The bounding rectangle of mapScene_ is now set to be mapQGraphicsView's
  childrenRect().
- curPoint_'s and curSpeed's ZValue is set to be 1 after
  adding new items.
MAP: POI's state changeable by clicking. Removal changed.

- POI's state can now be changed to red by clicking it and changed back
  to black by reclicking.
- Removal changed to apply to red POIs only
MAP: bugfix to clearRedPois

Erasing red POIs caused program to crash sometimes due to incorrect
pointer removal and delete order
MAP: added removable walls

- now by doubleclicking with right mouse button you can add the start and
  end points of a wall.
- the wall turns red with single click.
- red walls and POIs can be removed with button "Remove red objects"
MAP: reworked starting point and added "Reset angle"-button

- starting point can now be selected and removed. Removing it causes all
  tracking to be reset and removed from map except for angle_-variable. Angle
  can be reset with a dedicated button.
- some minor cosmetic changes and a tracing bugfix
MAP: added interactive scaling

- Map's width in the real world can now be set in cm and applied
  anytime.
- Roomba's triangle and trace is now at realistic size.
MAP: Fixed distance scaling