--keep-zoom-vp will no longer block the dynamic window resizing mechanism.

Closes: #382, #384

This introduces a new feh_should_ignore_image function which is called
at appropriate places in those modes to skip images which are loadable but
undesired.

Halves the start-up time of the slideshow if the title contains data from 'file->info'

This will (by default) check the image dimension on-the-fly, but you can still do it beforehand by specifying the preload flag.

Fixes issue 325.

like %o and %z in slideshow actions (I would like to use this to zoom in, pan,
and then use an action to crop the window to zoomed in view).

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)