Add the ability to auto detect supported brightness plugins

[ZeroErrors]

Description

Related issue: fixes https://github.com/openDsh/dash/issues/48

This PR adds the ability to automatically detect the supported brightness plugins.

When the config value for brightness_plugin is set to auto, which is the default, we iterate the available brightness plugins and check if they are supported and select the plugin that is the highest priority supported plugin.

Implementations:

• official rpi checks to see if /sys/class/backlight/rpi_backlight/brightness is writable and has the highest priority since it provides a true brightness control when available
• mocked is always supported
• x is supported if QGuiApplication::primaryScreen() returned non-null and we can successfully execute xrandr

Checklist:

• [x] The code change is tested and works locally.

[rsjudka]

i think one issue is that using the rpi screen (and in turn that brightness sysfile existing) does not necessarily guarantee brightness control will be supported (older versions of the screen dont support it). So that would be a case where auto will fail (not a major issue tho)

[ZeroErrors]

oh, wasn't aware that there was cases where it's not supported and the file would exist. I guess we can probably add extra checks when cases come up, if we can detect the display model or something.

[rsjudka]

hmm someone who is more familiar with the pis than myself can chime in, but I haven't heard of a way to detect the different version (v1.0 and v1.1) in sw