Fix GTK folder selection with filters.

[xStrom]

This PR disables file filters when opening folders, because the filter makes it impossible to traverse in the folder structure.

Fixes #947.

[luleyleo]

I think we should log a warning in case someone tries to do this.
Also I think it would make sens to either enforce this on all platforms, or add documentation that there is no guarantee for filters to work with folders on any platform (besides macOS)?

[xStrom]

After looking more deeply into it, filters don't really work on directories even on macOS. They work on packages instead. I cleaned up the API and added better docs in #960.

This PR now focuses on matching the newly documented API in behavior.

• Directories don't get filtered here, and they don't get filtered on other platforms.
• There's no logging of warnings, because specifying filters in a directory selection mode is legal. They are used to determine which paths are packages and which are directories. (The opposite of what was happening here.)

[luleyleo]

Considering #960 this seems reasonable to me.