Alternative HTML-based implementation for the overview

Provides a basic alternative HTML implementation for the
overview, that can be accessed at 'about:html-overview'.

https://bugzilla.gnome.org/show_bug.cgi?id=723950
Remove the overview widget and use the html based one instead
Add snapshot path to EphyOverviewStore

It stores the path were the snapshot has been saved and notifies with a
signal when a new snapshot is saved in the cache.
overview: Do not send decoded snapshots to the web process

Use the thumbnail path instead so that thumbnails are loaded in the web
process.
Notify the web extension about history changes to update the overview

And remove the automatic refresh when an overview item is removed via
javascript.