web-process-extension: register GearyWebExtension JS in the right place
We need to register our JS when the window-object-cleared signal is emitted. Previously we did this when the WebPage object is created, but this only worked due to luck. Since WebKit r271642, it no longer works. window-object-cleared is emitted right after page-created, meaning that right after Geary registers its JS, it all gets wiped away. Oops.
Fixes #1168 (closed)