Add configuration option to remove window shadows

[morgant]

When using a compositor such as picom with translucent shadows enabled, it'd be nice to offer a configuration option to remove the hard shadows that are rendered on windows by mlvwm. That said, the hard window shadows rendered by mlvwm are part of the look-and-feel, so they should be enabled by default.

[morgant]

Note to self: I was most recently testing picom with mlvwm as follows:

picom \
  -c \
    --shadow-radius=2 \
    --shadow-offset-x=0 \
    --shadow-offset-y=0 \
    --shadow-opacity=0.75 \
  -f \
    --fade-delta=5 \
    --corner-radius=1 \
  --vsync \
  --no-ewmh-fullscreen