x11: make screensaver code more compact, change DPMS handling

Reduces some code-duplication.

Just call DPMSEnable/DPMSDisable, instead of DPMSForceLevel when
reenabling DPMS. "Force" sounds evil, and messing with DPMS is already
pretty evil. I'm not even sure that we should.
client API: fix "missed" property notifications

If a property is notified as changed, and then again (before the change
notification is returned to the client), and the second change is a
sporadic change (i.e. nothing actually changed) and the change
notification was associated with with a data type, it could happen that
a change was "overlooked", because it would detect no change on the
second notification.

This is actually a pretty annoying corner case, due to the annoying way
we do things, so just store both the previously returned _and_ the newly
obtained property value. then we always compare with the user value to
check for a change, excluding any possibility of a missed change.

Note that we don't (can't/shouldn't) care if a value changes, and then
changes back; it's fine if that doesn't generate a notification. This is
due to how property notifications are supposed to be coalesced.
command: add a disc-menu-active property

Returns whether a DVD/BD menu is active. As requested by #788.
manpage: document discnav command