Remove asynchronous search in Dynamo

### Changes

This pull request removes the `DispatcherTimer` and the asynchronous SearchAndUpdateResults method in `SearchViewModel`.   Instead, all search is done synchronously.

### Results

I found search to be feel far more responsive, while also fixing issue #1092.  Notably, this removes any need to create a search Task or lock any data structures in the SearchViewModel class.  

However, it's possible we could run into performance issues on slower machines.  Is there a way to test this?  

@Benglin @ikeough