[alarms] Add functions for emitting DBUS signals about dialogs

DSME monitors the signal visual_reminders_status(int) from
interface com.nokia.voland.signal for dialog status information.
Signal parameter 0 indicates that a dialog is on screen, and
the value 1 indicates that no dialogs are shown. DSME uses this
information to turn the display on/off.
[alarms] Make dialog signaling more declarative
[alarms] Wrap voland signal adaptor to prevent overexposing