It is useful to be able to tell which layer you are selecting. The
checkboxes, on the other hand, provide no information when dealing with
a single layer selection
Improve dialog layout and fix UI policy issues with all of the dialog
boxes in the Gerbview and Pcbnew code paths.
Updated the use of component to footprint where appropriate.
Pcbnew: better test for allowed layers when creating/editing Dimensions and some other Graphic items
Drc: fix comments and messages for some drc tests.
To do: fix issues in active layer selection when creating a track and layer pair selection dialog