Ensure that InitialTargets and DefaultTargets from imported projects

Ensure that InitialTargets and DefaultTargets from imported projects
are honored.

In class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine:

	* Project.cs: Get InitialTargets from the imported projects also.
	The combined list of targets are executed, in the order they were
	found.
	DefaultTargets are also read in from imported projects, and the first
	non-empty DefaultTargets found is used.

In class/Microsoft.Build.Engine/Test/Microsoft.Build.BuildEngine:

	* ProjectTest.cs (TestInitialTargetsWithImports):
	(TestDefaultTargets):
	(TestDefaultTargetsWithImports):
	(TestNoDefaultTargetsWithImports):
	(TestNoDefaultTargets): New tests.

In tools/xbuild:

	* xbuild/Microsoft.Common.targets: Remove _ValidateEssentialProperties
	from BuildDependsOn, which was a hack. It will get invoked from the
	InitialTargets now.

git-svn-id: svn+ssh://mono-cvs.ximian.com/source/trunk/mcs@142653 e3ebcda4-bce8-0310-ba0a-eca2169e7518