Add a setting for not loading element hiding rules

[fhd]

Background

Not all clients are interested in element hiding - they could almost half libadblockplus' memory usage by not loading element hiding rules.

What to change

I see two API options:

1. Add a field to AppInfo, e.g. supportsElementHiding (suggested by Wladimir).
2. Add a setter and getter for a property, e.g. loadElementHidingRules.

I'd prefer the second variant. AppInfo is information about the app, while whether or not we load element hiding rules is more of a technical detail.