power: Fix inhibition management
We should:
- always have the same inhibit state for DPMS and screensaver;
- activate inhibition whenever an inhibit request is received or presentation mode is enabled;
- lift inhibition only when presentation mode is disabled and no inhibition request is active.
This change achieves this, simplifying and clarifying the code in the process.
Fixes: #210 (closed)
Supersedes: !55 (closed)
Edited by Gaël Bonithon