Check project/client name to avoid duplicates. closes #331

Added a simple method to check if name is valid at the moment of save, if not, show a warning message. 
Joey's implementation is missing and probably will have to wait for "material" re-design. 

fixes #331
Should use `.ConfigureAwait(false)` here and in the other extension method to prevent the next line to be run on UI thread. Less scheduling on UI thread = better.