Qt: Workaround to playlist single-click activation on Linux due to Qt

The activated() signal in Qt has platform-dependent behavior. On many Linux desktop environments, it is emitted on a single click, while on Windows it is emitted on a double click. This leads to an inconsistent user experience when interacting with the playlist.

This patch replaces the activated() signal with the explicit doubleClicked() signal for all playlist views (Tree, Icon, List, and Cover Flow). This change ensures a double-click is required to play a media item regardless to the OS.

I've also added a keyPressEvent to handle the "Enter" key, so the user can still play a media item by pressing "Enter," maintaining consistent behavior across all platforms and input methods.