Feature request: if a unit is defined, highlight it in the CellML-text

[agarny]

Ported from opencor/opencor#2853 by @mirams:

When working in CellML-text view with equations like this:

It would be nice if you could tell whether the units were all defined or not. As it is easy to make a spelling mistake that is hard to spot in a load of equations.

Couple of options:

1. colour all defined units the same purple as SI units,

   • pros: simple,
   • cons: I can see you might want to reserve special status for those.

2. colour all defined units a different colour?