* Fix assertions in Eeschema when editing net labels, electronic rules check,
and plotting.
* Fix assertion when opening PCB calculator.
* Fix assertions in Pcbnew when changing footprints in footprint properties
dialog and adding or editing text.
subclass DIALOG_SHIM support.
Tried dialog_drc but it has nesting problems due to use of DRCLISTBOX, which
needs to be put into its own header file to untangle.