* Network: Retain constraint values in label font handling
Fixes#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.
* Made unit test more linear, removed tabs
* Made 'enhanced subset' of unit test
* Removed TODO from comment
* Culled redundant nodes from unit test
wimrijnders
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