Support virtual paths when defining PropertyEditors via Attributes (U4-5128)

Previously you were not able to have your view start with `~/` when registering PropertyEditors through Attributes.  It worked fine when defining via `package.manifest`, but the [code to handle that](https://github.com/imulus/Umbraco-CMS/blob/a23fb7bc741df76376b640a4db5abe2fb92991fe/src/Umbraco.Core/PropertyEditors/PropertyEditor.cs#L102-L105) was not running for those defined via Attributes.

Fixes [U4-5128](http://issues.umbraco.org/issue/U4-5128)