Fix redactor dropdowns loose focus before the click event is fired when embedded into a non inline block dialog.

Hi,

I've found an issue when you have a block edit dialog (non inline) and a reactor editor within that dialog, when you select some text within the reactor editor and click a dropdown open (for example "Insert Link") then on mousedown the focus will switch to the top most input within the dialog, losing your text selection.

Steps to reproduce:
* Install my test block https://github.com/mlabrum/test-block/tree/master/test 
* Add the test block to an area
* when the option dialog displays select the text within the reactor edit
* click the Link button
* click "Insert Link" and notice the focus swaps to the top input and you lose your text selection.

Stopping propagation from the dropdown item prevents the focus loss.

Cheers,