Mouse support isn't really finished - the goto-dir dialog should have
ok/cancel buttons, and currently you can't interrupt file loading and
thumbnail updates. The latter two would need a custom mouse event
handler temporarily installed, so that we could avoid losing any
clicks. Actually it might not be too bad an idea to always use a
custom handler; that would be easier. After all, on a slow machine you
can already lose clicks during a file-selector screen redraw!


Generalise the read-from-a-filter mechanism used by the TIFF support
so support for other file formats can be added by the user if they
have a suitable converter.


File move should probably delete any existing thumbnail for the file
if the file itself is moved successfully.


It might be better if file delete were made to work in the
tagged-files-or-current-file-if-none-tagged way that copy/move do,
though I don't think I'd be hugely comfortable with that. Perhaps
there should just be an option to make it work that way.


(from readxpm.c)
>  * XXX one problem with this - >256-col reading is suboptimal for cpp==2.
>  * Could be made quicker by using same sort of speedup used for <=256-col
>  * files when cpp==2, i.e. by using a 65536-entry direct-lookup array
>  * containing ints (well, longs) with rgb values. That would be 256k,
>  * but that wouldn't be too bad to malloc.
