Support for overriding MSBuild global properties

[tintoy]

Extend the configuration schema to support specifying overrides for MSBuild global properties:

{
  "msbuildProjectTools": {
    "msbuild": {
      "globalProperties": [
        {
          "name": "MSBuildSdksPath",
          "value": "C:\\Foo\\Bar"
        },
        {
          "name": "AnotherProperty",
          "value": "HelloWorld"
        }
      ]
    }
   }
}

If these properties conflict with any existing definitions that would otherwise be supplied by the language service, then these properties will win.

Relates to tintoy/msbuild-project-tools-vscode#35 and Relates to #5