Make mega65-libc a submodule; cosmetic updates to SpritEd

[dansanderson]

Short-term cosmetic updates to SpritEd:

• Replace descriptions of unimplemented features in Help text with "(not impl)" (SPRITED: Implement "XWIDE" #77, SPRITED: Implement horiz and vert flip #78)
• Disable broken features: xwide (does nothing: SPRITED: Implement "XWIDE" #77), 50-line (crashes: SPRITED: Alt+D crashes #74), load/save (prompts for filename but does nothing; feature needs to wait until we've solved disk images mounted in subdirs)
• Improve the Copy prompt so it's clear that it's a copy to a sprite slot, expecting a number from 0 to 7

This PR also switches to using mega65-libc as a submodule, which is needed because months ago mega65-libc reorg'd and paths have changed. Without the submodule, the build depends on an older unspecified version of mega65-libc being in the parent directory.