Plan for spriteset import/export

Currently it is possible to import 4-bit BMP files as a single frame into the sprite editor, but this is awkward because the palette for a sprite set has to be the same, and managing palettes in most external graphics editors can be quite a pain.

The plan is to enable the import and export of full sets of sprites as a wide strip-shaped image with each frame arranged side-by-side much in the same way they are in the sprite editor.

• Write functions to get/set pixels within the Frame objects used by sprite_draw
• Re-write the mark/clone tool
• Re-write the copy/paste feature
• Write a command that can take a Frame object and write it to a 4-bit BMP
• Write a command that can load a 4-bit BMP and load it into a Frame Object
• Replace the export functionality with the new import-to-frame routine
• Add export frame functionality
• SUBify the individual sprite editor and rewrite it to work entirely with a Frame object with no screen-page storage
• Rewrite the sprite browser to work entirely with Frame objects and no screen-page storage
• Add full-sheet import/export functionality