Allow for update of existing scene

[scooper4711]

I would like the ability import to foundry to a new scene, fix things in DD, then update the existing scene in foundry.

[cube1us]

I second that - but also found some interesting behavior. If I re-import an updated DD map from DD under the SAME scene name (and DD exported file name) as the original, it creates a "duplicate" scene. Curiously, if I delete the duplicate [which may or may not be necessary], then exit FVTT and then restart FVTT, the ORIGINAL scene is updated. But that leaves me just a little uncomfortable that it might mess something up in the process. UPDATE: This is mostly a visual thing: the image .png is updated, but the new wall data (json) isn't merged -- not really a surprise there.