[xbuild] Add support for multi-targeting arbitrary frameworks.

ToolsVersion 4.0 supports targeting arbitrary frameworks defined
by FrameworkList.xml files and referenced by $(TargetFrameworkMoniker),
which is of the format:
 Identifier,Version=<version>[,Profile=<profile>]

built as:
 $(TargetFrameworkIdentifier),Version=$(TargetFrameworkVersion)[,Profile=$(TargetFrameworkProfile)]

eg. '.NETFramework,Version=v4.0' .

Frameworks are searched for in (order):

  1. Paths specified in the environment variable $XBUILD_FRAMEWORK_FOLDERS_PATH
  2. /Library/Frameworks/Mono.framework/External/xbuild-frameworks on Mac OSX.
  3. MSBuild property $(TargetFrameworkRoot)
  4. $prefix/lib/mono/xbuild-frameworks (default location)

.NET frameworks 2.0/3.0/3.5/4.0 have corresponding FrameworkList.xml
files now, which allows projects to target them. Custom frameworks like
MonoDroid can install suitable FrameworkList.xml files, and projects
can target them using the correct $(TargetFrameworkIdentifier) and
$(TargetFrameworkVersion).

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