PR #3308 moves two scripts from leo/plugins to leo/scripts

[Edward K. Ream]

PR #3306, added calls to  g.assertUi('qt') to picture_viewer.py and remove_duplicate_pictures.py. But that's a blunder: g.assertUi('qt') assumes g.app.gui exists. Oops: these files are stand-alone scripts that do not create g.app.gui.

PR #3308 fixes these mistakes by:

- Moving picture_viewer.py and remove_duplicate_pictures.py to the leo/scripts folder.

- Removing the calls to g.assertUi('qt').

- Improving messages when imports fail.

Summary

Please change any references to these two files.

Edward