Add a folder-tree entry type.
Move the data from mtpc-folder-tree.h to mtpc-file-data.h
List contents of MTP devices.

Finally, fix all the browsing issues with browsing the MTP device.
* uses a queue to stash the parent directory each time we load a directory
  or pop from the queue when we go up one directory.
* Uses MtpcFileData extensively. Eventually, we need to remove multiple
  implementation of the folder tree and use a generic one with MtpcFileData.