Add comments. Fix bug  #1351507 (pad color does not reflect, whether the pad is on front or back layer) and bug #1351388  (Adding a keepout area appears on the opposite layer than selected)

Remove a minor warning message in Debug mode for 2 dialogs.
Pcbnew: update Gerber X2 file format (File Attribute) to very last X2 specification
Eeschema: Fix a minor bug in block selection (sometimes the last selected component was select instead of items in selected area)
and better drag behavior when draging an item by the drag hotkey.

Introduced SELECTION_CONDITIONS to determine which menu entries should be visible in the SELECTION_TOOL context menu, depending on the selection.

Reworked the way of processing events in TOOL_MANAGER class.
Added GetCommandStr() for TOOL_EVENT class.

Introduced a new type of action: TA_ACTIVATE to distinguish events activating tools from common tool actions.

Reduced number of switched events, allowing to use VIEW_CONTROLS, even if there is no extra event dispatcher.

Event handlers are (dis)connected depending on the active view.
TOOL_DISPATCHER inherits from wxEvtHandler, so now it receives events directly instead of being fed by external handlers.

Minor changes (removed an unnecessary event handler and two function calls that had no real influence).

Moved View related routines & fields from PCB_{BASE,EDIT}_FRAME & BOARD to PCB_DRAW_PANEL_GAL.

Removed EDA_DRAW_FRAME::SetGalCanvasActive() [it may be misleading], added EDA_DRAW_PANEL_GAL::GetBackend().

switch back to original sexpr usage of PTREE, add new DSNLEXER constructor, enforce accessors in kicad.exe