[Monocle] CanLoadDocument() now returns LoadCheckResult.
[Monocle] Added IBackendPlugin::GetSupportedMimes().
[Monocle] Core::CanHandleMime().
[Monocle] Added IRedirectProxy.
[Monocle] Take redirectors into account in CanLoadDocument().
[Monocle] Added CoreLoadProxy.
[Monocle] Migrate to CoreLoadProxy.

Helps supporting async loading and redirections.