Add support for previewing images in room scrollback (#108)

src/tests.rs

@@ -172,6 +172,7 @@ pub fn mock_dirs() -> DirectoryValues {
         cache: PathBuf::new(),
         logs: PathBuf::new(),
         downloads: None,
+        image_previews: PathBuf::new(),
     }
 }
 
@@ -195,6 +196,7 @@ pub fn mock_tunables() -> TunableValues {
         .collect::<HashMap<_, _>>(),
         open_command: None,
         username_display: UserDisplayStyle::Username,
+        image_preview: None,
     }
 }