Only emit the marker-created event when the marker exists

This caused problems in the case of find-and-replace:select-all with 
multiple editors into the same file. 

* a marker is created on the TextBuffer capturing the selection
* DisplayBuffer:create-marker is fired from the first DisplayBuffer. 
The marker is turned into a selection which is merged into the current 
selection, deleting the marker that was created.
* DisplayBuffer::handleBufferMarkerCreated is called on the second
DisplayBuffer. The marker has been destroyed at this point, so it emits 
DisplayBuffer:create-marker with undefined. 
* ERROR>

Closes #3364