Added raising/lowering paths.  Added AddRemoveNodeTool.
Massive path-stuff rewrite.

Paths and Nodes no longer have global unique ID numbers.
Nodes belong to exactly one Path.
Path/Node member variables are private as they should always have been.
Tidy up path tool snapping behavior.

When creating or moving nodes, snapping occurs in order of increasing priority:
1) Snap to grid corners.
2) Snap to path segments.
3) Snap to nodes.

These need to be made into options, although snap-to-grid is toggle-able via
the Control modifier.
Added fit-to-width, fit-to-height, and stroke-width to texture dialog.
Added saving & loading of world textures and path texture info.
Fixed snap-to-grid when moving nodes.
Alt modifier toggles snap-to-segment for path tools.
Use the non-stroked path when hit-testing paths.