[darktable] Show image count for folders containing both images and subfolders

**Is your feature request related to a problem? Please describe.** In Collection module/folders each folder shows in brackets the amount of images it contains, a good thing. But it only happens for last level folders, folders that do not contain other folders. If a folder contains both, some photos and some folders, the amount of photos in that folder is not shown so they are not counted in the folders view. **Describe the solution you'd like** In Collection module/folders view, when a folder contains both, some photos and some folders, show in brackets the quantity of photos in that folder instead of showing no quantity at all (for instance, if the folder "Animals" contains 14 images plus some additional folders (like "Birds", "Insects",...) show in the view "Animals (14)" instead of just "Animals". **Alternatives** <!-- A clear and concise description of any alternative solutions or features you've considered. --> **Additional context** <!-- Add any other context or screenshots