about-handler: Do not send encoded images to the web process in about:web

Pass the icon filename instead.

https://bugzilla.gnome.org/show_bug.cgi?id=724862
about-handler: Do not send encoded images to the web process in about:applications

Pass the app icon filename instead.

https://bugzilla.gnome.org/show_bug.cgi?id=724862
Add ephy-resource:// custom URI scheme implementation

Ideally we could use resource:// URIs since they are supported by both
WebKit and libsoup, but the web process doesn't have access to the
resources compiled in the UI process. And in multiprocess mode, the
network process doesn't have access to the resources either.
ephy-resource:// URIs are like a proxy to load gresources compiled in
the UI process, the data is sent directly to the web process in
single process mode or to the network process in multiprocess mode.

https://bugzilla.gnome.org/show_bug.cgi?id=724862
about-handler: Use ephy-resource for the incognito icon in about:incognito

Instead of decoding the image and send it to the web process.

https://bugzilla.gnome.org/show_bug.cgi?id=724862
overview: Compile the missing thumbnail icon as a gresource

And use it directly form the css using ephy-resource URI.

https://bugzilla.gnome.org/show_bug.cgi?id=724862
about-handler: Compile the about css file as a gresource

And use a link to the resource in the about pages instead of including
the file contents in the about page.

https://bugzilla.gnome.org/show_bug.cgi?id=724862