Revamp searching in the file browser

[sakertooth]

This PR revamps the search functionality within the file browser, with the intent of improving the code and user experience.

Changes:

• The work of searching through the filesystem has been moved back onto the main thread. Since this is a blocking operation, we process Qt events asynchronously while searching. This reduces a lot of the complexity involved in the search code as there is no need for any extra thread or a Qt signal/slot connection, while still keeping the application running smooth.
  • Consequently, FileSearch was removed.
• Search results are no longer shown in a hierarchy, but instead as a flat list, with only the found directories being hierarchical. This aligns better with most file managers and greatly simplifies the search code (no need to convolute things by attempting to build a hierarchy). Since only the name of the directory or file that matches the filter is shown, path information has been added to the context menu for both items (as well as an "Open" and "Close" action for directory items).
• Searching no longer considers extensions. This is because extensions have no bearing on what kind of file a file is. This should open doors to allow finding for esoteric files that have unusual extensions but are perfectly working as they should, or files that are perfectly supported but have an odd extension.
• Searching now follows symlinks.
• Searching now supports finding hidden files if the "Hidden content" checkbox is enabled.

[sakertooth]

For some reason while searching, there is a segfault when closing the program. Doesn't seem to be a concern, but unsure why this happens.