dispatchContextMenu() stores the cursor position before displaying a
menu, so the tools use the original cursor position when processing
events (instead of the mouse position pointing to a menu entry). Later,
the previous position has to be restored, but if in the meantime the
tool was cancelled - previous settings were restored to a wrong tool.
Added REASON enum for Reset() function, so tools will know why a reset occured.
Fixed SELECTION_TOOL (it was bailing out, when a new board was loaded and some items were still selected).
Added removal of VIEW_ITEM groups after changing layers and removing items.
TOOL_ACTIONs can be run from CONTEXT_MENU after adding them.
Move tool actions are available to be run from CONTEXT_MENU displayed after right mouse button click on selected items.
Added some asserts to check the code.