element ruler: change ruler to be able to create multiple geometries while measuring

[astroidex]

Allows to draw multiple geometries. Generates calculation for every single geometry.

Please check whether the layout is ok. Any ideas?

Ideas for future developments

• highlight geometry on mouse-over
• calculate total (length or area of the geometries)
• allow to delete geometry from the list
• allow to modify geometries

[astroidex]

After discussion we decided not to take this enhancement in the core as some usability feature are not solved.