Import: Grouping: If we import something else than a jpg, and if a jpg i...

...s currently the group representative, change it to the new imported image.

This makes the grouping behavior a bit more consistent when importing new images. Like this the RAW/other file will become the group representative usually, and not alphabetically sorted: Sometimes the raw and sometimes the jpg.
For example for Nikon you currently get the jpg as group representative, for Sony Nex, you get the .arw raw file. This patch changes this. The case will become different though, when png, tiff or other file formats would get used. Maybe a priority list of file formats for grouping could be introduced for that ...