SourceCodeClassifier prefers Function category if there are several different ones for the same Range

It fixes wrong generated signatures colorization when some class members are got ReferenceType category.
Well, it turns out a workaround. For the `Prop` in this test https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/tests/FSharpVSPowerTools.Core.Tests/SourceCodeClassifierTests.fs#L588 FCS return `get_Prop`, `set_Prop` and (I guess) `v` - for the backend field:

![image](https://cloud.githubusercontent.com/assets/873919/4882215/235938b8-6353-11e4-9a9f-5f5320df7dae.png)

Accessibility of that `v` symbol is not public, so we get all its uses and FCS returns zero (not even one for the symbol itself). On that knowledge we make the decision that the symbol is not used. At the end of classification process it's assigned `Unused`, `Other` and `MutableVar` categories. If we amend the algorithm as you suggest `Unused` one will win and  `Prop` will become gray.