Kdalton/sample feedback

@anttikajanus - Please review. Updates were made to the MessageProcessingSample, SymbolDictionarySearch, DynamicLayerAddData, and LoadWebMapMVVM. 

It's good practice always check if the command can be executed before executing it ie.

````CSharp 
// meta code
if (LoadWebMapCommand.CanExecute(e.AddedItems[0]))
    LoadWebMapCommand.Execute(e.AddedItems[0]);
````