#39 with "ignore extensions" some files make gimv slow
when "ignore extensions" is set then gimv can become
unresponsive and use a lot of memory while trying to
decode large files which are not images.
there are several ways which can help with this:
(some of these may be usefull features on their own,
not only to fix this problem)
limit the time gimv tries to decode an image. if it
fails after a few seconds abort, and don't decode until
the user requests
limit filesize
don't autogenerate thumbs when entering a new directory
but only read from cache until user requests update.
cache a complete failure to decde a file so that it is
not tried again.
don't try to decode every file but use file magic
to detect if file could be an image.
don't hide non-images if extensions are used, just
don't try to decode them (it is important to see if a
directory is really empty (so i can remove it) or if it
only contains no images)
for the above if files remain without a thumbnail
decode such files only when the user selects them, and
manually requests to update thumbnails for that file.