Finish integrating Antonis's desktop_approots work

Finish integrating Antonis's desktop_approots work

In addition:

- moved video type detection into a separate file: videoTypes.dt out
  of datatypes.dt.  Also, removed all actions from Antonis' dt files,
  as these are handled by the new actions (listed below).

  Image, postscript, and PDF types definitions are still located in
  datatypes.dt.src

- supports most image files, PDFs, postscript files, and video files
  via dtapp now.

- Added new actions:

  - DisplayImage
  - DisplayVideo
  - DisplayPDF
  - DisplayPS

  All of these call the dtapp_* helpers to locate an appropriate
  program to handle the task.

- Added a camera icon

- standardized the icons used to display the various types - all
  images use the Dtimage icon now, as an example.

- moved the new *.dt files to their proper place in programs/types,
  rather than programs/localized/C/types (my bad).

Please add any further video types into videoTypes.dt, and everything
else (for now) into datatypes.dt.

In the future, we should probably separate these out into
type-specific .dt files.  Also, something should be done about playing
audio files too (maybe DisplayVideo.dt can handle all those too?)