Replace descriptions of unimplemented features in Help text with "(not impl)" (#77, #78)
Disable broken features: xwide (does nothing: #77), 50-line (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.
Short-term cosmetic updates to SpritEd:
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.