[3.0] Qt: Workaround to file explorer single-click activation on Linux due to Qt

The latest in the series of "little annoying things you have to fix, but no one ever does" …

(1st part : double click in the playlist : Qt: Workaround to playlist single-click activation on Linux due to Qt)

Problem: In the QFileDialog used by VLC, there is inconsistent behavior between Windows and Linux regarding folder navigation. On Windows, users need to double-click on a folder to navigate into it, while on Linux, a single click is sufficient. This inconsistency can be confusing for users switching between platforms.

Solution: Implemented a custom QProxyStyle that forces the QFileDialog to use double-click behavior on all platforms, matching the Windows behavior that users are accustomed to. This ensures consistent user experience across all operating systems.

The implementation creates a VLCFileDialogProxyStyle class that overrides the style hint for item activation, ensuring that double-click is required to open folders in the file dialog. The solution is applied to both the dialog and its child views to ensure consistent behavior throughout the entire file dialog interface.

This change only affects the file opening dialog and does not impact other parts of the application.

Tested on Linux/KDE.