Try a different name for loading libwebp.

Hopefully this will be a bit more robust - it is suggested by the Mono
docs.
Add an error dialog if we were unable to load the WebP library.

This requires an update of Pinta.Core, since the IImageImporter
interface needed to be changed to pass in the parent window.
Update the Pinta.Core dll
Merge branch 'master' into windows

Conflicts:
	WebPAddin/lib/Pinta.Core.dll