cache: store all cached data under XDG_CACHE_HOME

Both cache.json (article list) and the images directory are
regeneratable from the server, so they belong in XDG_CACHE_HOME
(~/.cache/net.jeena.FeedTheMonkey/) rather than XDG_DATA_HOME.

src/image_cache.rs

@@ -5,7 +5,7 @@ use std::path::PathBuf;
 use crate::model::Article;
 
 fn images_dir() -> PathBuf {
-    glib::user_data_dir()
+    glib::user_cache_dir()
         .join("net.jeena.FeedTheMonkey")
         .join("images")
 }