feature request: add optional filters to stop processing of some filesystem files

[rwperrott]

Make the filters addable, so can do things like ranges

Filters for each filesystem file, e.g. :

• file bytes size <, <=, ==, !=, >, >= to a numeric value
• file created timestamp <, <=, ==, !=, >, >= to a date or date-time value
• file modified timestamp <, <=, ==, !=, >, >= to a date or date-time value

Filters for each image in a format (like files in an archive, or images in a PDF), e.g. :

• image bytes size <, <=, ==, !=, >, >= to a numeric value
• actual image type is in a list of types, or not in a list of types.
• image file extension is correct or incorrect for its actual image type
• image height or width <, <=, ==, !=, >, >= to a numeric value
• image colour/gray-scale depth <, <=, ==, !=, >, >= to a numeric value
• image is or is not monochrome

Optionally allow failure of an image filter to stop processing of a filesystem file.

Possibly allow mouse selection of a file to show test result of filters in cbconverter-gui.