Add element hiding debug mode

Background

See eyeo/adblockplus/abpui/adblockplusui#608.

What to change

Add new elemhide_debug preference, and when set highlight elements targeted by element hiding and element hiding emulation instead of hiding them.

Hints for testers

As of now there is no UI to enable element hiding debug mode, but you can do so by running following code from any extension page (e.g. the options page):

browser.runtime.sendMessage({type: "prefs.set", key: "elemhide_debug", value: true});

Elements targeted by element hiding (and element hiding emulation) should not be hidden but highlighted in a shade of red with this preference set.

Also make sure that element hiding filters are still logged correctly in the devtools panel (with and without this preference enabled).