Fix alpha cut-off handling at level 0

Instead of performing a complex cutoff operation, it now just converts
the layer to 24bpp.  Much simpler.
Temporary adjustment to multipage TIFF/ICO/GIF handling during batch processing

I haven't decided on a good way to handle these, so for now I am
implementing a strict "only process first page/frame" limit.
Suppress image load errors during batch processing
New support for automatic alpha thresholding during batch processing

Previously, a user dialog was required.  Now, the function can simply be
passed a value (previously acquired from the user)
Minor preference form layout tweaks
Myriad batch process tool bugfixes and feature implementations

All file format options should now be present and working, with the
exception of JPEG-2000 quality which I will add tomorrow.