Image thumbnail bar is now resizable

Yay!  Thumbnail bar scroll implementation at present is not great;
specifically, it is difficult to resize once the scroll bar appears.
But until I decide on an "official" scrolling solution, I'm not wasting
time improving this one.

Also, I've suspended the interface warning for now.  After a few of my
best testers give it a run, I may reinstate nightly builds.  Things
should be "stable enough" at this point... (fingers crossed)