Sandbox: this application does not receive implicit persmissions

Currently, using Flathubs QOwnnotes is disallowing use of directories outside of the initially proposed one (`~/notes`). I suppose this is a consequence of it being sandboxed. However, file dialogs should be granting [implicit access](https://docs.flatpak.org/en/latest/sandbox-permissions.html) if a user navigates somewhere outside the sandbox. Currently, the open directory dialog just does nothing if a custom directory is being chosen, without feedback as to why. From my understanding, this is incorrect behavior (and quite limiting :) ).

A fix would be much appreciated!