Add support for folder favorite action

Signed-off-by: Mario Danic <mario@lovelyhq.com>

src/main/java/com/owncloud/android/files/FileMenuFilter.java

@@ -243,14 +243,14 @@ public class FileMenuFilter {
         }
 
         // Favorite
-        if (!allFiles() || synchronizing || allFavorites()) {
+        if (!canBeFavorited() || synchronizing || allFavorites()) {
             toHide.add(R.id.action_favorite);
         } else {
             toShow.add(R.id.action_favorite);
         }
 
         // Unfavorite
-        if (!allFiles() || synchronizing || allNotFavorites()) {
+        if (!canBeFavorited() || synchronizing || allNotFavorites()) {
             toHide.add(R.id.action_unset_favorite);
         } else {
             toShow.add(R.id.action_unset_favorite);
@@ -316,6 +316,10 @@ public class FileMenuFilter {
         return mFiles != null && !containsFolder();
     }
 
+    private boolean canBeFavorited() {
+        return mFiles != null;
+    }
+
     private boolean containsFolder() {
         for (OCFile file : mFiles) {
             if (file.isFolder()) {