Ignore all project item types apart from None and TypeScriptCompile.
This fixes the startup exception when opening SharpDevelop.Tests.sln
with SharpDevelop when the TypeScript addin is enabled.

System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at ICSharpCode.TypeScriptBinding.Hosting.TypeScriptContextProvider.AddFileToProjectContext(TypeScriptContext context, FileName fileName)
   at ICSharpCode.TypeScriptBinding.Hosting.TypeScriptContextProvider.CreateProjectContext(TypeScriptProject project)
   at ICSharpCode.TypeScriptBinding.TypeScriptProjectMonitor.CreateTypeScriptContextIfProjectHasTypeScriptFiles(IProject project)
   at ICSharpCode.TypeScriptBinding.TypeScriptProjectMonitor.SolutionLoaded(Object sender, SolutionEventArgs e)

Fixes icsharpcode#773

Fixes icsharpcode#780

The SlowCheetah NuGet package uses the MSBuild API to directly modify
the MSBuild project file stored in memory in MSBuild's
GlobalProjectCollection. It adds properties that contain special
MSBuild characters, for example $( ). When SharpDevelop was merging
these new properties back into the project file on disk it was using
MSBuild to encode them so the wrong values were added. Now the
property values are not encoded.

…site (and very popular)