bump up theme version, add rpi backlight plugin, implement onLoad plugin method

[gregd72002]

• changes default-theme from 89995dfcf329fbac2f5d4dad6231572a5bf7ffd0 to 3021b80a85c6ae2e0c7d47d0ff289524bc0079e0

• adds onLoad method to plugins (invoked once settings are loaded)

• implements rpi backlight plugin

The following changes are also needed to the headunit-gui so it installs to target PREFIX: https://github.com/gregd72002/headunit-desktop-rpi-imagebuilder/blob/dev/package/headunit-desktop/004-gui_installdir.patch

[viktorgino]

Thanks, will test it on my Pi tonight and merge this.

[viktorgino]

Thanks. This pull request has been integrated manually.