Patch several small issues with MathObjects

These are several small items from the forum or bugzilla that I've fixed.

1.  Use parentheses around a complex number when it is subtracted from something (so `z-(1+3i)` rather than `z-1+3i`).

2.  Have `Formula->cmp(diagnostics=>1)` use `$PG->debug_message()` rather than `warn` so that its HTML output will be properly formatted.

3.  Fix `ImplicitPlane` object so that it can be used properly in Lists.

This should also be merged into `develop`, either now, or by merging `release/2.8.1` back into `develop` at the time it is 2.8.1 is officially released.