libgui: Fonts can specify alternate families for different weights

However, at least on OS X 10.9 it seems Qt is currently unable to
select a light font no matter which mechanism is used.
UI|libgui: Disable widget background blurring when a game is loaded

Game rendering is simply not efficient enough to be done multiple
times per frame => perf hit is too big.
Default Style|OS X: Use Open Sans as UI font (like on Windows)

Standardizing appearance. Also, Qt is having real difficulties
picking the right font weights on Mavericks, which makes things
look ugly with Helvetica Neue. Let’s hope that future updates will
fix this.
Cleanup|Log|libgui|libappfw|Console: Log message cleanup
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine
API: Relocated internal thinker functions from public API header

These belong in an internal header.