adblock: Postpone navigation decisions until ready

This defers navigation decisions while EphyFiltersManager is performing its initial setup, with the aim of avoiding touching the network without the adblocker filters being in use. This is particuarly important to avoid accidentally making requests to trackers which would be filtered by the adblocker in the small time window while the EphyFiltersManager initializes.

Blocking of requests is done by tapping into WebKitWebView:decide-policy because the signal can be handled asynchronously, and while a decision is not made WebKit will not start performing requests. Thus, a list of pending decisions is kept and items which were added to it are decided when the EphyFiltersManager:is-initialized property changes.

The existing VerifyUrlAsyncData struct (which was being used by the safe browsing code) is reused to hold the information about pending navigation decisions.

Closes #872 (closed)