Webclient: cursor shapes (#552).

In various situations a proper cursor shape is shown:
- Hovering a topic does not show a text cursor (but leaves the default cursor).
- While something is moved a grabbing hand cursor is shown. Applies to topic moves, canvas moves, and cluster moves.
- When a topic's type icon is hovered a crosshair cursor is shown, suggesting an association can be drawn.
- While an association is drawn a crosshair cursor is shown.

Note:
- Some situations are only supported in conjunction with the DOM based Box Renderer plugin.
- Some situations are not supported in Safari 5 (MacOSX 10.6.8).

See #552.