Make drop shadow optional for mouse cursors

Drop shadows for mouse cursors can be now turned on or off (Preferences / Interface).

Internally new option has been added (cursor-drop-shadow next to the existing cursorscaling) to control drop shadows in cursors. All cursors have been modified to include CSS and point to single filter definition.

Setting cursor-drop-shadow to 0 in preferences.xml will turn drop shadow off. The drop shadow itself has been delegated to a separate file (cursors-drop-shadow.svg) and its intensity has been dialed down. The goal is for all cursors to share the same filter definition (Hopefully filter auto-size is going to adopt on the fly).

The node tool cursor had its tip trimmed to see if it looks better at low-res displays. To be determined. It does not look better on high-res.