Use a global "mouse accuracy" setting

Should have done this sooner.  This fixes a high-DPI problem where pixel
amounts were being used to calculate "is mouse over object", so on
high-DPI displays, extraordinary precision was required.

Mouse accuracy is now unified across the project, and calculated
independent of screen DPI.
Ongoing attempts to make jcButton DPI-aware

VB is partially to blame, as its handling of various user control events
(such as Resize) is hugely inconsistent.

The preview control now forcibly redraws the jcButton toggle after
sizing events, which fixes inexplicable clipping that was happening on
high-DPI displays.

I've also made various jcButton calculations DPI-dependent, though that
hasn't solved all DPI issues.  Still trying to figure out why some text
calculations are off...