Move media attachment long-click event to context menu.

Long-click on a media attachment will now bring up the normal
context menu for a ConversationItem long-click, but with the
addition of a "save attachment" option.

This allows users to long-click on messages with media in them
and still see the other contextual menu options.

// FREEBIE

src/org/thoughtcrime/securesms/mms/SlideDeck.java

@@ -74,5 +74,15 @@ public class SlideDeck {
   public List<Slide> getSlides() {
     return slides;
   }
+
+  public boolean containsMediaSlide() {
+    for (Slide slide : slides) {
+      if (slide.hasImage() || slide.hasVideo() || slide.hasAudio()) {
+        return true;
+      }
+    }
+
+    return false;
+  }
 	
 }