[ewiki] upload-filenames plugin

There is a new plugin (plugins/feature/imgfile_naming.php), which
chooses the original filename in favor of a md5-sum string (the
internal:// prefix will however still be used). It requires a
complete ewiki update (CVS tarball).
It discards stupid names like "DSC00001.jpg" and "test-1.gif" and
can use the DestPageName as fallback for constructing a better
database storage name.
(I had also liked to use images` comment fields as fallback, but
most ones only carry an even more useless "Created with the GIMP"
inside, which then made no good filename either.)

#

The image upload mechanism is also likeley to change, in that
the picture file will be transmitted in one <form> (together with
the <textarea>), so no target="_WINDOW" and JavaScript will be
necessary in the future. The speed experience my however suffer
from such an approach, and this part cannot be made configureable.

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