Unify the minimum CMake requirement for all platforms.
Enable C++11 support.

This will make it impossible to build GammaRay on Mac OS X 10.6 with its
GCC 4.2. But it'll allow us to use some C++11 features unconditionally that
are available in MSVC 2010 (the new lowest supported compiler), such as
lambdas, rvalue references (except on *this) and auto.
Add support for registering multiple signal spy callback sets.

Qt only supports a single one, which was used by the TimerTop plug-in so
far. Instead we now have a list of callback sets in the probe, and a single
registered callback that just dispatches to that list of callbacks.

This allows additional plug-ins to use the same feature, such as Mathias'
signal tracer.