Mouse prefs: Layout improvements

* Some spacing was uneven leaving items uncentered
* Removed "slow/fast" labels from the sliders as they were not correct
(a delay is short/long, not slow/fast, same for acceleration) and not
very useful.

Signed-off-by: Adrien Destugues <pulkomandy@gmail.com>
Mouse prefs: further layout fixes.

* Use BSeparatorView instead of trying to replicate it with a BBox
* Use B_USE_SMALL_SPACING instead of 7.

Fixes #11528.
Outsource VL-Gothic font.

Fixes #11576.