libdeng2: Added HighPerformanceTimer, refactored Time to use it (and pimpl)

de::Time can now internally use a simple time value from the new
HighPerformanceTimer. This is a simple "seconds since launch" (ddouble)
as opposed to the complicated Date/Time pair that de::Time uses
by default.
Optimize|libdeng2|App|GuiApp: High-performance timing for the App/Animation Clock

It is pointless to calculate animations using time values that include
date information. Now the GUI application's clock uses the simpler
high-performance timing.

Removes use of the expensive QDateTime::msecsTo().
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine