[xbuild] Override global properties before project Load.

[xbuild] Override global properties before project Load.

MSBuild task can override properties for a project, and these
should be applied before the project file is loaded. This is to
allow the properties to be available for Imports. Eg.

 <Import Project="$(SolutionDir)\foo.targets"/>

$(SolutionDir) is generally set by the MSBuild task in a .sln.proj
file.
Bug reported by JB Evain.

    * Microsoft.Build.BuildEngine/Engine.cs:
    Apply global properties before the project load.

    * Test/*/EngineTest.cs (TestGlobalPropertiesImport*): New tests.