Store DRC settings in board file

[ubruhin]

Summary

This adds the DRC settings to the board file to make them persistent (see #669). Due to the needed refactoring, I also moved the settings from the DRC dialog into the new Board Setup dialog. As the DRC dialog is no longer useful, I removed it. The corresponding menu item / shortcut now opens the DRC dock widget and immediately executes the DRC instead of just opening the DRC dialog.

Note that the checkboxes to enable/disable each check individually have been removed as I think it makes no sense to store this information persistently. I'll open a PR soon to provide an alternative way to execute only parts of the checks.

File Format

 (design_rule_check
  (min_copper_width 0.2)
  (min_copper_copper_clearance 0.2)
  (min_copper_board_clearance 0.3)
  (min_copper_npth_clearance 0.2)
  (min_annular_ring 0.15)
  (min_npth_drill_diameter 0.25)
  (min_pth_drill_diameter 0.25)
  (min_npth_slot_width 1.0)
  (min_pth_slot_width 0.7)
  (allowed_npth_slots single_segment_straight)
  (allowed_pth_slots single_segment_straight)
 )

Screenshot

Closes #669