[MSBuild] Properly serialized null DependentOn metadata

Fixes ungrouping files.
[Ide] Remove search for new files on project load

It's unmaintained, buggy and has poor UX.

BXC17832 - Using the "Search for new files on load" and "Automatically
include found files" options with a large (thousands) number of files,
the build action is not properly set for all files.
[Core] Add API to get define symbols from project

Added DotNetProjectConfiguration.GetDefineSymbols. Subclasses can
override it to provide symbols defined automatically by the build process.
[Core] Simplify SupportsFramework for PCL projects
[Core] Fix TargetFrameworkVersion in project templates

It now works for frameworks woth than .NETFramework.