Added a few settings to make the layer settings more similar to KiCad defaults (moved vias to VIA_THROUGH_VISIBLE, added a few rules for required layers).

Selection clearing is invoked using TOOL_ACTION object rather than its name.

TOOL_EVENT message is supposed to contain string as parameter.
Added missing header for class_drawsegment.h (KiROUND).
Renamed SELECTION_TOOL::containsSelected() to SELECTION_TOOL::selectionContains().

Removed a few RecacheAllItems() calls, some of them changed to specific type recaching (using TYPE_COLLECTOR & VIEW_ITEM::ViewUpdate() ).
Removed OPENGL_GAL::SetStrokeColor().

Fixed a missing group clearing and update for restored VIEW_ITEMs.Fixed a missing group clearing and update for restored VIEW_ITEMs.

- VIEW_ITEM::ViewUpdate() does not update items immediately. Now it marks them to be updated and the real update occurs on the next rendering frame.
- VIEW::InvalidateItem() made private.
- VIEW_LAYER::enabled -> visible
- Some functions moved to header files.

Most of the GAL tools are activated by corresponding buttons on the right toolbar.
Removed menu entry for PNS router (as it is activated as the usual "Add track" tool).

gr_basic.cpp: rewrite the function which draws the outlines of a thick segment.

Removed an unnecessary header from class_board_item.h.