fixed drag & drop behavior of lighttable.

1. drop image(s) in left pane:
the image will be displayed in the left pane

2. drop image(s) in right pane:
the image will be displayed in the right pane

3. fill light table (F6):
the first image of the added images will be displayed in the left, the second in the right pane. All the others are added to the thumbbar.

4. append images to light table (CTRL+F6):
Images are only appended to thumbbar, not displayed automatically.

5. drop images in thumbbar:
Images are only appended to thumbbar, not displayed automatically.