Improve UX of filter field

Romain Talon reported that the filter field is cleared when you select an image. This MR fixes this issue and more (like not clearing the fitler when navigating back to the parent folder) by revamping the behaviour of the filter field.

Now, the file browser remembers the filter string independently for each directory:

Additionally:

• I fix a minor Bootstrap regression with the little button to clear the filter: the button was disappearing when the field received focus.
• I now always show the hint that says how many entries are hidden/filtered out (previously, it was only displayed when all entries were hidden).

Before: After: