fix isFeatureSelected performance

[timnyborg]

isFeatureSelected() becomes a performance bottleneck when editing layers with 30,000+ features, because it's running indexOf on every single feature (so O(n^2) time, regardless of how many features are selected).

With this change, each feature is checked against the contents of the selectedFeatures list, which is O(n) if you have a single item selected, gradually taking more time as the list increases. This could probably be improved further by using a Set for selectedFeatures, but I don't know the implications of altering the structure of the layer's state.