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)

Removed TA_ActivateTool (now tools are invoked by sending TA_Action event, with the tool name as string parameter).
Developed TOOL_Action class & added ActionManager. Hot keys registered by tools are processed.
Selection & move tool can be invoked by a hot key.

Fix minor bugs in cleanup dialog options and plot solder mask function (thanks to Lorenzo to locate these bugs)

Fix minor bugs in cleanup dialog options and plot solder mask function (thanks to Lorenzo to locate these bugs)

* Remove fp_lib token from board and s-expr netlist file format.
* Fix a bunch of Doxygen warnings.
* Remove some extra debug log messages.