Re-factor handler for `WM_SYSKEYDOWN` events

The handler for `WM_SYSKEYDOWN` events underwent re-factoring to promote
a _smaller_ reliance on repeated conditional testing and a decrease in
scope level.

The swap from multiple `if`-style conditionals to a single `switch`
statement allows simplification of deep scopes. With the use of a
`switch` comes a decrease in the maximum scope level. The number of
required tests on the value of `wParam` also drops.

The expression passed to `ICVar::Set` is also modified to more directly
indicate the _flipping_ of the value.