treeview: Fix horizontal scrolling.

[Duane Clark]

In beersmith, two things were preventing the drawing of scrollbars. 
First, the main check was whether the tree width was greater than the 
window width, ignoring whether the tree was in fact already scrolled. 
The second thing was that the horizontal scrollbar was only being 
redrawn if the scroll position changed. But other things, such as the 
size of the window changing (which beersmith did), can require that the 
scrollbar be redrawn.

Changelog: Fix horizontal scrolling.
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