HeadsUp: added option for Importance threshold
Allows to suppress heads up for notifications that are of low importance
(such as download completed, google now, ...). Importance threshold is
configurable allowing 3 levels: low, medium, high; including possibility
to completely turn off importance check.

For UNC, importance level is used for "Default" heads up mode thus won't
affect special heads up modes such as "Always", "Immersive"