Fix GIF/TIFF/ICO loading to load all frames/pages/icons as individual layers

GIMP actually has a pretty nice wizard for multipage images, where you
can select individual pages to load (or any combination of pages), and a
choice for loading pages as layers or individual images.  Something like
that would be nice to add to PD, but in the meantime, multipage images
are at least working again.

Also, I've made a note to look at multipage exporting once we add an
export menu.  Exporting a layered file as a multipage TIFF would allow
for importing the layered file into something like Photoshop or GIMP, at
least in rasterized format.
Layer toolbox: fix vertical scroll bar resetting at unwanted times