Only save feature properties when actually needed

[perliedman]

We store feature properties in our layers for use when a layer is interactive and/or can change feature styles after they're loaded. However, when this feature isn't enabled, storing the properties is unnecessary and can take up a lot of memory.

This PR makes sure properties are only saved when we actually need them.

[jkuebart]

Note that other than through getFeatureId(), properties on layers are also visible to event handlers such as click. Only making them available if getFeatureId() is specified might break some client code.

Maybe the condition options.getFeatureId || options.interactive might be strong enough?

[jkuebart]

Adding some unit tests for this project might be worthwhile, and I'd be willing to write one for my assertion above, but I'm afraid setting up the framework from scratch is too daunting to me…

[perliedman]

You're right, should check interactive as well. It sort of hightlights that the interactive and getFeatureId aren't 100% orthogonal: I have a hard time coming up with a use case where you would set interactive: true but not supply getFeatureId.

[jkuebart]

In my case, clicking a feature triggers an update in a separate window without the need for any feedback in the map itself. I can work out which feature was clicked from its properties, but since I don't have a need for setFeatureStyle() I don't supply getFeatureId.