Also remedies places where the video backends and core rely on things
being indirectly included.

And rename it to VideoBackendBase because of conflicts within the backends itself.

Also gets rid of global headers

This reverts commit 81414b4fa2edadf0b2a2bb0bd4df61dd59165eec, reversing
changes made to b926061f641692ae31bacbeba304f5941db7d3bc.

Conflicts:
	Source/Core/DolphinWX/Frame.cpp
	Source/Core/VideoCommon/VideoConfig.cpp
	Source/Core/VideoCommon/VideoConfig.h

It reduces redundancy. Needed make possible to save current video config.

The new implementation has 3 options:
 SyncGpuMaxDistance
 SyncGpuMinDistance
 SyncGpuOverclock

The MaxDistance controlls how many CPU cycles the CPU is allowed to be in front
of the GPU. Too low values will slow down extremly, too high values are as
unsynchronized and half of the games will crash.
The -MinDistance (negative) set how many cycles the GPU is allowed to be in
front of the CPU. As we are used to emulate an infinitiv fast GPU, this may be
set to any high (negative) number.

The last parameter is to hack a faster (>1.0) or slower(<1.0) GPU. As we don't
emulate GPU timing very well (eg skip the timings of the pixel stage completely),
an overclock factor of ~0.5 is often much more accurate than 1.0