Pcbnew: Fix Bug #1420074 (Crash when a footprint is added to the board (or edited) from the footprint editor, if a new pad was added, because the net info of the new pad is broken in this case)

  Committing this separately from the rest to ease making a patch for just this
  functionality in case that it might make it into the trunk.
  
  This can function standalone, though it is less useful without the rest.
  It requires that wxwidgets-3.0.0_macosx_magnify_event.patch be applied to wxWidgets
  It is completely optional; everything is guarded by the USE_OSX_MAGNIFY_EVENT macro.
  
  - Added OnMagnify event handler to EDA_DRAW_PANEL, EDA_3D_CANVAS and the helper for
    EDA_DRAW_PANEL_GAL, WX_VIEW_CONTROLS. This should cover canvases all current tools.
  - Guarded all with USE_OSX_MAGNIFY EVENT feature macro and added support in CMakeLists.txt

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.

Moved menuCopy to CONTEXT_MENU copy constructor.

Resolved HOME hot key conflict between menu entry (Zoom Page) and event assigned to the hot key in the ACTION_MANAGER.
Conflicts:
	pcbnew/menubar_pcbframe.cpp