Hidden content is shown when "Hidden content" checkbox is unchecked

### System Information

Arch Linux

### LMMS Version(s)

Probably has been here for a long time

### Most Recent Working Version

Probably has been here for a long time

### Bug Summary

Directories in the file browser still show hidden content, even when the "Hidden content" checkbox is unchecked. This does not happen at the top level, but when searching through the directories.

This happens because the `Directory::addItems` function does not take into consideration if the entry is hidden and should or shouldn't show the entry, unlike in `FileBrowser::addItems`, which does take this into consideration. Solving this bug properly could mean a lengthy refactor, though a quick and dirty solution might be a "hidden" flag in each directory object that is updated via a Qt signal on changes to the "Hidden content" checkbox.

### Expected Behaviour

Entries inside directories should not show hidden content when the "Hidden content" checkbox is unchecked.

### Steps To Reproduce

In the file browser (any one of them works), navigate to a hidden directory inside another directory. The hidden directory will be shown regardless of the checked state of the "Hidden content" checkbox.

### Logs

Click to expand

### Screenshots / Minimum Reproducible Project

_No response_

### Please search the issue tracker for existing bug reports before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by reading Directory::addItems and compare its hidden-entry handling with FileBrowser::addItems. Reproduce the issue by navigating into a directory in the file browser with the "Hidden content" checkbox unchecked. Done means hidden entries remain excluded inside nested directories while visible entries continue to appear.

Written by the indexing model from the issue text.