FIX: check the output directory validity
clean up, rename bool_settings for general
FIX: #67 Allow show hide columns

Now we can show hide certain columns on load time, this is done using
the settings value "columns_only_show" with the possible values separated
by spaces:

id = unique number identifier
ext = file extension
name = file original name
size = image size widht x height
output = file output name, as artscript will be naming them
osize = output size or sizes
color = original image colorspace (sRGB mostly)
path = complete image location path

This also fixes the settings "autor" not beign taking into account.