Pcbnew: Add concept of a 'point'

This is a zero-dimensional object that can be used for snapping
and documentation of useful, but non-physical features on a
board or footprint. They do not correspond to any physical
output in exports or plots.

Points do have a "size", but this is a graphical property only
and determines how large they are drawn on the board.

They also have a layer, which allows them to be selected and
made visible according to layer filters.

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/4691

pcbnew/tools/array_tool.cpp

@@ -298,6 +298,7 @@ void ARRAY_TOOL::onDialogClosed( wxCloseEvent& aEvent )
                     case PCB_DIM_RADIAL_T:
                     case PCB_DIM_ORTHOGONAL_T:
                     case PCB_DIM_LEADER_T:
+                    case PCB_POINT_T:
                     case PCB_TARGET_T:
                     case PCB_ZONE_T:
                         this_item = item->Duplicate( true, &commit );