daemon: Refactor internal brightness step adjustment API
Split out of !361 (merged) to avoid piling all the changes into that MR.
As we move toward multi-display support, the existing
screenBrightnessKeyPressed() API isn't good enough anymore.
Brightness changes could come from either D-Bus API and for any
display, so it's not enough to check for a function result with
the new brightness of just a single display.
Instead, provide a new adjustBrightnessStep() function that does
not return any values, but instead forwards an IndicatorHint enum
value to brightness change signals. This allows us to show the OSD
from BrightnessControl's brightness change handler, and keep it
working when another action requests the percentage indicator
to be shown.
In the process, also clean up some of the naming in BrightnessLogic.