Make grid dropdown list configurable (lp:#1806143)

Original report created by eelik (eelik)

The list of grid pitch values in Grid dropdown/menu is quite long. I very
seldom use more than maybe 6 values and finding those values in the list is
tedious. It would help if the list were configurable. There could be a list
of checkboxes and values in a configuration dialog. Only the checked values
would be visible in the list. That would also make possible to add more
values to the hardcoded list, like 0.02mm and 2mm which now aren't there
(although there is 0.2mm). Also 0.001mm (the minimum) could be there.

Original tags: pcbnew ui