fix issue where dblclick wouldn't clear selection

This PR removes the check for drag.start/endPercentage before triggering drag-clear. dblclick-to-clear didn't seem to work, since by the time the dblclick would occur, the drag.start/endPercentages seemed to always be null. I may be missing something with respect to the refactor (nice job on that, btw @katspaugh). This seemed like a regression to me, so I went ahead and fixed it in this commit. 

I'm not sure if the integration with the ELAN plugin was was part of the motivation for the null checking, but it seems to play nice with that plugin after this fix.