Bug 687388 - Visual Studio project generation; r=mshal

A new moz.build-based build backend for Visual Studio project generation
has been added. The build backend can be used by specifying
'VisualStudio' to the backend option of config.status or mach
build-backend. e.g. `mach build-backend -b VisualStudio`.

Usage docs have been added to build/docs/visualstudio.rst.

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extra : rebase_source : dfb5c43a22434600c5bb4870ec5be2f39b4820e9
extra : amend_source : f1000754400f280778a669b8c34d90c5ef95d966