Created Updater class for handling application updates and moved
respective API from FilterEngine to Updater component.
For now Updater class does not care about asynchronicity (if any)
in JS files which are used for update checks logic.
TODO: Move Updater class to dedicated .h and .cpp files.

Abstracted jsSources from FilterEngine. Now every component (like
FilterEngine and Updater) created by Platfrom gets EvaluateCallback
and evaluates own JS files needed for internal logic.

EvaluateCallback keeps track of evaluated files to avoid duplicated
evaluations.

Moved Updater class to dedicated header and source files.

Created apiUpdater.js with a separate API for Updater component.

Issues to fix:

1. Pass Prefs to Updater component
- Now Updater sets just an empty Prefs object

2. Review content of apiUpdater.js and updaterJsFiles[] array
- Check that all what's needed is set there and nothing is redundant
- Does Updater component need to take care of asynchronicity in JS
files?

3. Fix UpdateCheckTests
- They are now passing just because FilterEngine object is created
while those tests should just rely only on Updater object.

Added cpp methods for Prefs JS API.
Removed not needed JS API.
Added _isSubscriptionDownloadAllowed event callback to unblock
downloads in JS code.
Cleaned up UpdateCheckTest a bit removing FilterEngine because
with the change above those tests don't require anymore
FilterEnigne object to pass.

Apart from changes in new code, this also withdraws
changes in existing code not related with this feature.

Now creation of Updater is thread safe.
Added synchronization for evaluatedJsSources set.
Added more unit tests.