Pcbnew: better dialogs to select the active layer or a layer pair. Add an option (hotkey+popup menu) to place a via and select the new active layer
(useful for boards having more than 2 layers)
Eeschema:fix compatibility with old schematic files, when they  contain ERC markers.
Pcbnew: better test for allowed layers when creating/editing Dimensions and some other Graphic items
Drc:  fix comments and messages for some drc tests.
Fix minor bugs in cleanup dialog options and plot solder mask function (thanks to Lorenzo to locate these bugs)