Custom quick access sections

[fluxxcode]

This PR implements FileDialog::add_quick_access and FileDialogConfig::add_quick_access, which allow you to add your own quick access sections to the left sidebar.

Example:

FileDialog::new()
    .add_quick_access("Project", |s| {
        s.add_path("☆  Examples", "examples");
        s.add_path("📷  Media", "media");
        s.add_path("📂  Source", "src");
    })