Can now adjust the magnitude limit using mouse wheel or "-" and "=" keys by
using the Alt key modifier. The Ctrl and Shift modifiers work in a similar
fashion to what they do with zooming.

The default mag step is 1.0.

The Shift modifier reduces the step to 0.5.

The Ctrl modifier reduces it to 0.2.

Combined, they reduce it to 0.1.

Also changed the factor in the maglim formula from 4.444 to 3.5 to match
the summer branch.