Many image collections are organized by directory, so it is nice to have
jump-to-adjacent-directory navigation.

e.g. Given the following file hierarchy:

    .
    ├── A
    │   ├── 1.jpg
    │   ├── 2.jpg
    │   └── C
    │       ├── 1.jpg
    │       ├── 2.jpg
    │       └── 3.jpg
    └── B
        ├── 1.jpg
        ├── 2.jpg
        └── 3.jpg

`feh --recursive` creates the following filelist:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

If we press [next_dir], we move the current_file pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg <-- current_file
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [next_dir] again moves the pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

[next_dir] now moves the pointer back to the top of the list:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [prev_dir] from here moves backwards to the first image of the
previous directory:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

When starting from an position that is not the first image of a
directory, [prev_dir] moves the pointer to the first image of the
current directory.

These actions combine well with `--sort dirname` since all regular files
in a directory will be sorted before any subdirectories, avoiding a
filelist like the following:

    A/1.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg
    A/4.jpg

With `--sort dirname` that filelist becomes:

    A/1.jpg
    A/4.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg

manually changed in collage, imlib, filelist, thumbnail, slideshow,
index
feh_display_status also changed

Patch by sdaau on IRC. Thanks!

Problems so far:
* leaks zombie processes
* does not work when terminating feh with a signal (since the convert process
  is no longer in feh's process group)

This was broken by the changes allowing --reload to retry ad infinitum.

Now force_new is used to decide whether or not to work around the Imlib2
caching issue.  Because of this, a few feh_reload_image calls needed to be
changed.

Patch by livibetter

This commit removes the feh_file_remove_from_list call from feh_reload_image.
We can safely do this because if the file itself was removed, cb_reload_timer
will take care of updating the filelist, and in all other cases it is intended
to leave the image in the filelist and retry.

A warning will be displayed in the image if the reload failed.

This reverts commit 8b79bc33.

This fixes a segfault in --reload when the image becomes unloadable while feh
is running.

Note that this does not affect --reload with more than one file: If the
current image is removed, feh will simply jump to the next image. feh will
only quit if the file still exists, but can no longer be loaded.

In the image_remove function, slideshow_change_image needs to be called before
removing the image from the filelist.  Because of this, the "x of y" displayed
by --draw-filename is wrong (y is 1 too high).

This commit introduces a new 'render' argument to slideshow_chingae_image.
The image_remove function calls it with render=0, edits the filelist and then
runs by winwidget_render_image itself.

Patch by Yu-Jie Lin.

gib_imlib_save_image_with_error_return was used, but it tries to set the Imlib
image format by examining the filename to save to.  However, it only supports
lowercase file endings (as in .jpg) and breaks with others (like .JPG).

Since giblib is unmaintained, for now the fix is only used here.