fix Bug #1345193:  Drill Map coordinates alway 0 when using Gerber format (due to a missing plotter initialization).

Apply a workaround to fix a wxWidgets  bug in versions 3.0.x relative to mouse wheel event (Windows specific).
Therefore wxWidgets 3.0.1 can be used without need to patch it.
(This bug is fixed in wxWidgets 3.1)

Gerbview: allows x.7 format (recently indroduced in Gerber format)
Also minor coding style fixes.

Gerbview: allows x.7 format (recently indroduced in Gerber format)
Also minor coding style fixes.

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.