Unix|qmake: Added build option for using ccache

“deng_ccache” now makes the build use ccache for faster compilation.
Merge branch 'master' into oculus-rift
UI|Client: Added shadow behind ProgressWidget’s text

Make it more readable on light backgrounds.
libdeng2|Log: Use (v) to identify verbose messages
Unix|qmake: Using ccache with gcc

The previous options were only suitable for clang.
Unix|qmake: Using ccache with gcc

The previous options were only suitable for clang.
UI|Client: Adjusted shadow of ProgressWidget text
UI|Client: Don’t recreate canvas during busy mode

It appears to be a little risky to recreate the Canvas during busy
mode’s secondary event loop (crashes in OS X sometimes). Now the
recreation only happens after busy mode has ended.
Merge branch 'oculus-rift'
Merge branch 'master' into low-latency-input