Refactors the way music folders are handled

- Second part of the fix for tsquillario/Jamstash#212 - Fixes getIndexes
- Moved loading / saving / deleting the selected music folder to persistence-service.
- Moved the request to get all the music folders to subsonic-service.
- Moved SelectedMusicFolder to subsonic-controller's scope from the rootScope
- Fixes a bug where we would send a '-1' music folder id to Subsonic, causing an SQL error when choosing "All Folders" and refreshing the Artists list.

app/common/persistence-service.js

@@ -77,6 +77,19 @@ angular.module('jamstash.persistence', ['angular-locker',
         locker.forget('Volume');
     };
 
+    /* Manage selected music folder */
+    this.getSelectedMusicFolder = function () {
+        return locker.get('MusicFolders');
+    };
+
+    this.saveSelectedMusicFolder = function (selectedMusicFolder) {
+        locker.put('MusicFolders', selectedMusicFolder);
+    };
+
+    this.deleteSelectedMusicFolder = function () {
+        locker.forget('MusicFolders');
+    };
+
     /* Manage user settings */
     this.getSettings = function () {
         // If the latest version from changelog.json is newer than the version stored in local storage,