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
There shouldn't be a need for a cast...
```C#
var existWithName = await dataStore.Table<ProjectData>().ExistWithNameAsync (nameEditText.Text);
```

Should work.