Implemented a pixbuf cache mechanism.

Since gdk_pixbuf::Pixbuf is refference counted and every episode,
use the cover of the Podcast Feed/Show, We can only create a Pixbuf
cover per show and pass around the Rc pointer.

GObjects do not implement Send trait, so SendCell is a way around that.
Also lazy_static requires Sync trait, so that's what the mutexes are.

hammond-data/src/models/queryables.rs

@@ -513,6 +513,11 @@ impl From<Podcast> for PodcastCoverQuery {
 }
 
 impl PodcastCoverQuery {
+    /// Get the Feed `id`.
+    pub fn id(&self) -> i32 {
+        self.id
+    }
+
     /// Get the Feed `title`.
     pub fn title(&self) -> &str {
         &self.title