Update Text Templating addin for SharpDevelop 5.

GAC references in text templates are currently not supported
unless they are referenced by the project.
Fix null reference exception when opening .txt files.

TextContentCondition now handles files that have no syntax
highlighting defined.
Fix pre-processed text template not generating a file.

Return code dom provider from C# projects.
Text Templating addin no longer using WorkbenchSingleton.
T4 template now handles project not returning a CodeDomProvider.

Default to using C# code dom provider and add warning to Errors
window instead of failing to pre-processing a template.
Add Text Templating addin to installer.