Changed reaction of SELECTION_TOOL to ToolCancel event (first event clears selection, second one deactivates the tool).

Moved VIEW_ITEM::ViewGetRequiredLayers() functionality to the VIEW class. Now required layers are set per layer basis instead of per item.

pcbnew: hooked Tool Framework into the edit panel. Added a sample selection tool (not fully functional).

Minor code cleaning. Pcbnew: better iniatilization of members in DRAWSEGMENT, TRACK, EDGE_MOD (useful when using  python scripting).