Disable brightness controls for systems using external displays

[ayoungethan]

As per https://github.com/elementary/wingpanel-indicator-power/issues/163

on 5.1.7

The brightness control settings are irrelevant for systems using only external displays. Suggest that both the display brightness slider and the "automatically adjust brightness" button be dimmed out/disabled when only external displays are detected, to avoid the appearance of hardware control capacity that does not exist.

This is similar, eg, to behavior that disables software control of bluetooth when no bluetooth device is connected.