Improve filter on import

This commit adds intelligent handling for the filter to be applied to the lighttable view upon importing images. The commit message below describes it in more detail:

Instead of showing all images, we set the filter to the smaller of the default import rating and the current filter. This ensures that the imported images are shown, but that we don't reduce the filter unnecessarily.

This is my first time (ever!) editing and submitting a pull request for C (or any compiled application) code, so go easy. ;-)