If tool was previously active and it is called again, it is brought to the top of the active tool stack.
It fixes issue of dragging of items that have EDIT_POINTs, when dragging was activated by hovering over an item and using hot key.

- resetting relative coordinates
- switching units
- not official: changing transparency for layers (works only for main layers, does not work on vias/pads/etc.)

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).

Added PCBNEW_CONTROLS for handling miscellaneous hot keys (trying to be compatible with the default hot key settings). Currently there are a lot of stubs to be filled out (in pcbnew_controls.cpp).
Handled actions:
- switching layers
- switching grids
- changing track width & via size

Refactored:
- EDA_DRAW_FRAME::SetNextGrid() & SetPrevGrid()
- PCB_BASE_FRAME::SetFastGrid1() & SetFastGrid2()
Removed a warning from RN_DATA::updateNet(int).

ROUTER_TOOL events have temporarily changed their hotkeys assignment.

Minor fixes (check for net codes size for ratsnest, cursor coordinates are not always shown as if the cursor was snapped).

Screen size is saved in VECTOR2I instead of VECTOR2D.

Removed EDIT_CONSTRAINT::Update() - replaced with resetting constraints.
EDIT_CONSTRAINT is stored in EDIT_POIN using boost::shared_ptr instead of pointer.
Added EDIT_LINE::GetEnd() & GetOrigin(). Overridden ApplyConstraint() for EDIT_LINE.
Side EDIT_POINTS for zones are drawn as circles.