Added MarkupExtensionModelTests for testing various strings being used with both shorthand and element style markup extensions.
Fixed a bug in MarkupExtensionPrinter to make the tests pass; it now handles spaces and backslashes correctly when printing shorthand markup extensions.
Added a simple test to verify that file path strings is parsed correctly.
Merge remote-tracking branch 'remotes/origin/WpfDesignerMarkupExtensionPrinterFix'