Fix dropout link creation on Firefox

Symptom:
Firefox returns wrong selection - not related with current caret position.
https://github.com/HackerWins/summernote/blob/develop/src/js/editing/Editor.js#L350 tries to set focus to editable before getting a range, but it doesn't work properly on Firefox.

This fixes https://github.com/HackerWins/summernote/issues/606 but implemented by very ad-hoc approach.