[xbuild] Microsoft.Common.targets - import Before/After targets files.

[xbuild] Microsoft.Common.targets - import Before/After targets files.

With this, Microsoft.Common.targets will import
$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\$(MSBuildThisFile)\ImportBefore\*
at the beginning of the file and
$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\$(MSBuildThisFile)\ImportAfter\*
at the end of the file.

So, this would import all files from (eg.):
$prefix/lib/mono/xbuild/4.0/Microsoft.Common.targets/ImportBefore/

This can be disabled by setting the bool properties -
$(ImportByWildcardBeforeMicrosoftCommonTargets) and
$(ImportByWildcardAfterMicrosoftCommonTargets).