Allow native theming with Qt-based Linux environments (e.g. KDE)

[MMachado05]

As of my writing this, the filesystem that appears when loading and saving projects is using the very barebones default Qt theme, which gives the program a very dated feel on modern Linux distributions. As far as I'm aware, there are ways to theme these pop-ups to better match the available native themes, especially with the AppImage format. I think changing this so that the Mu pop-up filesystem better matches the native filesystem would provide a much slicker end-user experience for Linux users.