Doc++

Remove redundant docs about desktop-GL-only functionality, proper
crossreferenes for overloaded functions.
Prefix plugin test libraries also with Magnum*.

They need to be installed into possibly system-wide location on Windows
and thus we need to avoid name clashes (or at least explicitly show that
e.g. TgaImporterTestLib.dll belongs to Magnum and is not any OMG virus).
package: strip all crosscompiled binaries.
Workarounds for platforms without std::to_string().

I hate this.
Merge branch 'master' into compatibility

Conflicts:
	src/Magnum/AbstractShaderProgram.cpp
	src/Magnum/CubeMapTextureArray.h
	src/Magnum/Shader.cpp