Support XDG FileChooser portal

• I have read the specified guidelines for issues

Is your feature request related to a problem? Please describe.

One of the problems with the Linux desktop is file pickers. Whenever we use a(n old) GTK app on Plasma, we get the GTK file picker, and vice-versa.

Tenacity uses the GTK file picker, so matter what DE you use, it will use the GTK file picker.

Describe the solution you'd like

I'd like to suggest supporting XDG FileChooser portal. This will make the app open the file picker of the DE you are using, which will make it a lot more consistent to the desktop and more familiar.

It's also really beneficial for Flatpak users, because this will let us remove filesystem=host permission (read write permissions to your whole system), which will make the Tenacity Flatpak infinitely more secure than what it is now. filesystem=host is as secure as installing a native package, which means it's inherently insecure; letting it access your shell configuration files, GPG and SSH keys, passwords, etc. is extremely risky and generally shouldn't be advisable generally, whether it's FOSS or not, because security vulnerabilities exist and can be exploited. Sadly, Tenacity is an audio editor, so it's crucial to easily access your files as well.

I'd also suggest supporting other portals whenever possible. This will improve the upcoming flatpak by a lot because we'll be able to remove filesystem=host permission and fully harden the application.