Image tabstrip: fix broken optimizations during resize

Horizontal resize of a horizontally-aligned tabstrip should no longer
require regeneration of thumbnails.
Metadata browser: swap in an API scrollbar to allow for very large metadata lists

Fixes #152.  Many thanks to @audioglider for reporting.

This is really only a temporary fix, as the dialog is going to be
largely rewritten once metadata editing is implemented.  But I don't
know if that will make the cut for 6.6, so in the meantime, this at
least fixes crashes on images with tons of metadata in a single
category.
Primary toolbox: minor fixes

The window can now be shrunk to the size of a single button, and padding
has been reduced to allow the window to shrink further.