Add ray line drawing and other "relative" drawing mode support

[Blackadder]

Currently JOSM does not have any traditional drawing capability, which makes it difficult for instance to draw objects or locate a point a specific distance from another point. Ideally it should be possible to do what a basic CAD package can do. Including:

1. Place a node at a specific lat/lon
2. Place a node a specific distance in metres (or optional feet) from an existing node at a given magnetic compass bearing.
3. Take relative metric co-ordinate entry from a given node (like an AutoCAD @4,7 type relative co-ordinate entry)
4. Support for right-angle snap from an existing segment (helps draw regular shapes)

These would all help translate basic surveying in the field into OSM nodes for objects that are difficult to obtain with GPS (or where GPS relative accuracy gives poor results, eg when tracing a building shape)

[Blackadder]

This list of features was not well formed:

1. Place a node at a specific lat/lon

2. Place a node a specific distance in metres (or optional feet) from an existing node at a given magnetic compass bearing.

3. Take relative metric co-ordinate entry from a given node (like an AutoCAD @4,7 type relative co-ordinate entry)

4. Support for right-angle snap from an existing segment (helps draw regular shapes)

[imi]

list reformed in description (in trac-wiki-syntax, put a space before list items)