Use the new WebKit content filters API for the adblocker

There's a few things missing and currently after I have just rebased today on top of master saving the sidecar files is failing; so the JSON rule set files are always downloaded. I'll be ironing out the rough edges in the next days but I think this is more or less ready for initial feedback.

Needs WebKitGTK+ built with the patches for the following two WebKit bugs:

• #193622 - [GTK][WPE] Add content extensions support in WKTR and unskip layout tests.
• #167941 - [WPE][GTK] Enable support for CONTENT_EXTENSIONS.

You will need to change the URLs for the adblocker rule sets to make them point to a JSON-formatted resource. For example:

gsettings set org.gnome.Epiphany adblock-filters "['https://better.fyi/blockerList.json']"

Closes #288 (closed), #602 (closed), #237 (closed), and #361 (closed).