Conditionally add features to the selectedfeatures of a layer when using selectFeature of ModifyFeatureControl

Previously everytime that `selectFeature()` was called (e.g. in standalone mode) it immediately added the passed feature to the `selectedFeatures` array of the configured layer. In our usecase we had a custom `SelectFeature` control, which also modifies that `selectedFeature` array, so that after calling `selectFeature()` of the `ModifyFeature` control we end up with duplicates.