TODO file:
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SPEEDUP fixes:

1. SPEED: Cache the object info for all items on the device.
  Right now, ptp_getobjectinfo() is called repeatedly on the same
  objects during startup, track listing, file listing, playlist listing,
  album listing and whatever we implement tomorrow. A lot of useless
  communication can be saved by cacheing this info. Notice that this
  needs to be updated whenever flush_handles() is called too.

2. SPEED: Cache the supported object properties at first read/startup.
  then use the cache to check for supported props instead of calling
  out to PTP with ptp_mtp_getobjectpropssupported() every time.
  The cache would be an array of size params->deviceinfo.ImageFormats_len
  with a list for each format of the properties it will support. Notice 
  that this needs to be updated whenever flush_handles() is called too.

3. SPEED: Cache track metadata, file metadata etc, perhaps this is not 
  desirable since the application using libmtp will do this anyway.
  (libgphoto2 does it, see http://svn.sourceforge.net/viewvc/gphoto/trunk/libgphoto2/camlibs/ptp2/library.c?r1=9491&r2=9590)

4. SPEED: Whenever we add an object (file, track, playlist...) we
  should only need to update the cache with relevant data. Atleast for
  speed caches 1 and 2 above.


FEATURE fixes:

1. FEATURE: Make abstract playlists really become size -1 when created as 
  the ones created on the device instead of the current 1 byte size.
  (Is this possible using enhanced commands? See TODO remarks in
  the create_abstract_entity() function)

2. FEATURE: Support playback and volume setting on devices that have it.
  (I don't have one that does - Linus.)

3. FEATURE: Make an API that can return several devices and let the user 
  choose which one to operate, not just connect to the first one...

4. FEATURE: Support relevant events. MTP devices seen in existance provide
  events for "object added" and "object deleted". These should result in
  atleast a call to the flus_handles() function.
