image_view: Add directory navigation

This commit adds directory navigation to Image Viewer.
Currently it only sorts images by name, instead of following
directory sorting like the mockups.

src/util.rs

@@ -0,0 +1,23 @@
+use gio::prelude::*;
+use glib::translate::*;
+
+pub fn utf8_collate_key_for_filename(filename: &String) -> String {
+    unsafe {
+        from_glib_full(glib::ffi::g_utf8_collate_key_for_filename(
+            filename.to_glib_none().0,
+            filename.len() as isize,
+        ))
+    }
+}
+
+pub fn get_file_display_name(file: &gio::File) -> Option<String> {
+    let info = file
+        .query_info(
+            *gio::FILE_ATTRIBUTE_STANDARD_DISPLAY_NAME,
+            gio::FileQueryInfoFlags::empty(),
+            gio::NONE_CANCELLABLE,
+        )
+        .ok()?;
+
+    Some(info.display_name().to_string())
+}