Re-enable system file chooser

We disabled the default system file chooser dialog in #15. However, this was mostly a temporary solution for https://github.com/flathub/org.scummvm.ScummVM/issues/14#issue-977565620 and as @ssokolow pointed out in https://github.com/flathub/org.scummvm.ScummVM/issues/14#issuecomment-905510268 , the system (xdg portal) file chooser actually allows granting permanent permissions to a specific directory.

However, there are some downsides of using the system file chooser as pointed out in https://github.com/flathub/org.scummvm.ScummVM/issues/14#issuecomment-905515872 + it would probably break things for current users. We should investigate this.