Implement support for border-box and font metadata

[rozniak]

Rzxe only crudely supports drawing sprites straight from an atlas. Support should be added to give additional context about the kinds of things that can be drawn by the atlas, notably 'border-boxes' and 'fonts'.

In this sense, an atlas file should contain information for this kind of metadata - I call them 'resources':

• Border-boxes should reference 9 sprites used to draw a box with a border (4 corners, 4 sides, 1 body), possibly metadata about stretching/tiling the sides/body
• Fonts should contain references to sprites for each character, metadata regarding kerning and character/line spacing

This has been started in dev-packer-assets, I have mainly been working on a single dialog for this (to do sprite mappings) though will need to think of a neat way to include the type-specific metadata properties.

[rozniak]

I think these may be too distinct from each other to implement in one clean branch - I think this should be done as separate features.