Slider UC: replace text box with text up/down

This FINALLY provides a way to fine-tune slider values.  Apologies that
it took so long to fix this.

Since I was updating the UC anyway, I've tried to improve the way
spacing is handled within the slider UC.  Previously, some hard-coded
values from the original scroll bar days were still present.  While this
new version makes much better use of available space, it also messes up
alignment in certain dialogs - but since we'll be making a lot of other
UI changes this release cycle, I'm not gonna worry about it just yet.