Fixes for hole clearance and hole-to-hole tests.

1) Separate out CONSTRAINT types
2) Filter both source and dest pads/vias for drilled holes when doing
   hole-to-hole checks.  We were checking the items being put into the
   DRC RTree, but not the items we were scanning.
3) Add hole clearance to Board Setup Constraints panel.

Fixes https://gitlab.com/kicad/code/kicad/issues/6546

Fixes https://gitlab.com/kicad/code/kicad/issues/4683

pcbnew/dialogs/panel_setup_rules.cpp

@@ -237,6 +237,7 @@ void PANEL_SETUP_RULES::onScintillaCharAdded( wxStyledTextEvent &aEvent )
                      "length "
                      "hole "
                      "hole_clearance "
+                     "hole_to_hole "
                      "silk_clearance "
                      "skew "
                      "track_width "