Add additional image count just for current directory

[Kevin-Mok]

Right now the count is just based on all the images stored in the HashMap. I think it'd be nice to be able to see both counts, although the ListView may end up becoming a bit cluttered. It'd actually probably more useful just to show this count with the images currently under the root directory, but it'd be more work to filter them out.

It'd also be nice to have a question mark icon with a tooltip saying what the count means.