Fix many doxygen warnings (due to missing info, old comments, typo ...) when building the doxygen doc.

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.

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.

Moved menuCopy to CONTEXT_MENU copy constructor.

Key events are handled by wxEVT_CHAR instead of wxEVT_KEY_[UP|DOWN]. Fixed issue of chars that require modifiers (e.g. ? is Shift+/ on US keyboard layout).

Hot keys specific to a tool have priority over global hot keys (TOOL_ACTION scope: AS_GLOBAL/AS_CONTEXT is finally handled properly).

Added possibility of switching tools by the right toolbar buttons, without deactivating the current tool first (previously tools did not switch if there was one active).

Refactored code that handled zooming events.
Added PCB_RENDER_SETTINGS::Get/SetSketchMode().
PCBNEW_CONTROL reacts to hot keys changing display modes (sketch via/tracks, high contrast).

ROUTER_TOOL events have temporarily changed their hotkeys assignment.

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 an unnecessary header from class_board_item.h.