Separate Locking from Blanking

Closes: #457 (closed) , prereq for #283 and phoc#206 (closed)

This is has man open ends but since there's movement in #283 let's give this visibility.

TODOs:

• blank screen again on screen lock (currently we either blank or lock)
• make power button behavior consistent (blank when screen lock is disabled, lock when screen lock is enabled after lock-delay) (needs phoc!407 (diffs))
• unblank on arbitrary keypress when blanked but not locked
• handle lock-delay (amount of time to lock after blank)
• Don't blank right away on ALT-L (rather lock and blank after short delay (as if locked from the power menu)
• check for screen-savers's active property rather than phosh_shell_get_locked () when figuring out feedback for notifications
• check other phosh_shell_get_locked () call sites (and locked property binding)
• Address primary vs build-in monitor TODOs

Rebased version of !747 (closed) so we can remove another stale branch from the main repo. Not worth testing yet since this is just a rebase (I have some more tweaks for this pending clean up).