Fixes almende#3517.

Due to changed logic in the label font handling, the option values for `widthConstraint` and `heightConstraint` were overwritten.
The fix is in effect a reversal of two code lines: parsing constraint options should come *after* parsing (multi)font options.

Further changes:

- Additional 1-liner fix: constraint values were not copied for edge-instance specific options.
- Small refactoriing of `Label#constrain()` in order to separate concerns
- Added unit test for regression testing of this issue.

This leads to the curious observation that, while the actual change is two lines of source code, this resulted in a +-150 line regression test.