SplitButton: Add loading/loadingFailed properties

[TommyLindh2]

New feature motivation

I have a "create new dialog" that have a create button and some extra save actions that does some extra things to the created object. While the saving happens it would be nice to support the loading state (Adding a spinner on the button) and also support, success/fail state so that you can handle exceptions/successes. So it's easier for the user to understand what happens.

New feature description

I would want the limel-split-button to have the same loading support as a regular limel-button i.e. adding the properties: loading and loadingFailed that behaves in the same way as in the limel-button