Trade: got rid of opening input streams in AbstractImporter.

Now it is possible to open either pure data arrays or arrays specified
with filename. The function calls might be ambiguous now (i.e.
open("image.tga") would open data array instead of handling it as
filename), renamed them to avoid that. Default implementations now
fire an assertion. Bumped plugin interface string version as this is
pretty incompatible change.
Trade: deinlined all AbstractImporter virtual functions.
Trade: updated AbstractImporter documentation.

Clearly state that deleting the data is user responsibility.
Treat image data as array of unsigned chars.

Arithmetic on void* is not defined, this helps a little with various
conversions.
Base for image converter plugins.