check for open exide instance

[bwbohl]

currently a click on the edit icon always opens a new instance of eXide. It would be better to check for an open instance first an open the file in the respective instance, as e.g. when opening a file from the eXist-db dashboard collection browser.

[bwbohl]

cf. https://github.com/eXist-db/dashboard/blob/master/plugins/browsing/browsing.js#L574