Blender Git Loki

Fix T74552: Distribute negatives in number input

This applies a relatively simple solution for fixing some unintuitive
cases in unit handling.

Currently entering -1m50cm evaluates to -0.5m, and similarly 1'6"
evaulates to just half a foot. So effectively there's an implied + just
between the numbers, which is quite confusing.

This works by adding parentheses so the negative distributes to the
block of values before the next operator.

For example:
| Before | After |
| `-1m50cm + 1m -2m50cm` | `-(1m50cm) + 1m -(2m50cm)` |
| `-4m + 0.5 / -1.1` | `-(4m) + 0.5 / -(1.1)` |
| `-1'6"` | `-(1'6")` |
| `-1e-2cm` | `-(1e-2cm) ` |

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D7813