Relative image path

Sprite sheet saves out the working image's relative path to "imgpath"
attribute and uses this to load the image, negating the need to have the
image in the same dir.
git ignore file
bunch of changes

- Sprite sheets and anim sheets now saved as .xml files
- Support for import and export of keyframe nodes in animations (no
graphical support)
- Animations exported as "simple" or "compound". Simple anims are those
that contain one frame per cell and on export, we ignore the Cell xml
tags and just output a bunch of <spr> tags instead (Cells are inferred).
- Suggested name for custom node changed to number of children, instead
of using unique ID
- Grid aligned sprite sheets can be quickly constructed using new editor
tools.