Feature : display cursor coordinates

- new controller CursorCoordinatesController
- added div in right column (bottom:0)
- 3 new events : CURSOR_MOVED, DRAG_START, DRAG_END
- modified tools to fire events when necessary

The cursor coordinates are displayed when the mouse is hovering the
drawing area. When the mouse leaves the area, the indication disappears.
If the user is using a tool that involves dragging (selection, rectangle,
circle), the indicator displays the original coordinates (captured during
drag start) and the current coordinates.
Merge branch 'master' of https://github.com/juliandescottes/piskel