Hook for non blocking slow validators

[andreasjakobik]

In addition to a way to contribute live validator that gives quick response, there must also be support for validators that take long time to complete (tens of seconds in worst case). These slow responders must not block any other live validation that is going on. They can take their time, and when done should update the editor just like any other validation does. Examples of these long lasting validations include:

• Check for uniqueness of namespace among registered Yang files
• Check for backward compatibility, which requires access to remote repository
• Compilation of yang files using target configuration component

These slow validators should perhaps only be triggered when file has been saved, or by an explicit validate action is called by user?

[svenefftinge]

Maybe we can combine this with #61 by providing a generic action contribution.