- Remove music dir prefs
+ Add refresh album art option to album list context menu
+ Wire up album view context menu
+ Force an idle after transport commands
+ Add "clear cache" button
* Use the OperationQueue's `operationCount` function instead of keeping
count ourselves. This is reliable now each command is entirely self
contained.
* Rename `queueCommand` to `enqueueCommand`
* Move the command Enum into its own model file
* Move the `enqueueCommand` function into MPDClient+Command