Settings: Introduce PhoshSettingsSlider
Using a common class in PhoshSettings
for sliders (currently volume / brightness) allows to add an optional button to go to power settings / sound settings and adding the torch slider (https://source.puri.sm/guido.gunther/phosh/-/tree/torch-slider, #386 (closed)) even easier.
Filing this here in case somebody wants to get started with something fairly confined.