Change slewed gauge display to low-pass filtered gauge display.

[Aeasala]

Filter time constants are derived based on the already assigned {minMaxTime} of a gauge object, where it is assumed that 5*tau ~ minMaxTime. The time constant, tau, is derived as such, for a discrete-time first-order LPF:

y[n] = y[n-1]*(1-a) + x[n]*a
    where y[n] is the new output, y[n-1] the current output, and x[n] the current input
a = {GAUGE_LPF_SCALAR * dt * 5.0/minMaxTime}, clamped to range of [0,1]

There is no divide-by-zero protection for the {minMaxTime} element, however, there was none before.

Should all gauges seem too sluggish/fast, there is a global scalar which can be applied to the response rate of every filter.
def {GAUGE_LPF_SCALAR} in {toggleswitch.h}. A higher value will yield a faster response.