A line object (with extobj::role=edit, on any layer) can be a basis of a dimension line.
The dimension line will stand out above the base line and will print the
length of the base line. As the base line is moved or resized, the dimension
line follows. The user can drag&drop move the dimension line to determine
its distance from the base line. The dimension line can be drawn on any layer.
1. optional: create a new extobj layer group and layer. When done, this
can be used to avoid the edit-object interfering with the board.
By user convention the new layer group should be a global doc
layer and the purpose should be 'extobj'.
2. draw the line
3. use the menu or {e s x} to convert the line into an extended object
4. edit the geometry by grabbing the edit-object line or its endpoints
5. edit properties by right clicking on the 'E' mark
6. the format string is a coordinate-only pcb-printf format string;
the %m formats are documented in src/pcb-printf.h