Implement WebKit2 authentication handling
There's no need to store the credential twice
Support cancelling a request
Disable hiding deprecated GTK symbols

GtkAction is deprecated in 3.10, but it is used in the webkitgtk
headers, so we can't force them off.
Merge branch 'dev/webkit2-authentication' into mathstuf/next

* dev/webkit2-authentication:
  Disable hiding deprecated GTK symbols
  Support cancelling a request
  There's no need to store the credential twice
  Implement WebKit2 authentication handling