FileFilter: make matching child files/dirs more precise

- When we match child files/dirs of the current dir, either the item name
is the "filename", or if it's a child-dir/somefile, then filename starts
with "child-dir + '/'".

To test create a file foo and a dir bar with a bar-file inside it then:
  - `ls`, foo and bar should be underlined
  - `ls bar/*`, bar/bar-file should be underlined
  - type bar..., before it would be underlined, after it won't be

- Capture QString by reference in lambda.