All: press ctrl+shift key when moving the mouse allows the graphic cursor to be moved outside the grid.
(useful to place graphic objects, texts in any position, regardless the current grid)

* Fix grid select box update bug on context menu.
* Fix via size and track width select box update bugs.
* Fix layer pair indicator button update bug.
* Fix auto track width tool bar control enable bug.
* Fix via size and track width select status bug in context menu.
* Fix layer select box and layer control widget select bug when current
  layer is removed.
* Add virtual function to notify objects derived from EDA_DRAW_FRAME that
  the units setting has changed.
* Coding policy class naming fixes.