Clang: Significantly improve ClangFormat plugin usability

What's new:
1. New LibFormat option is used to prevent lines shrink,
   which allows to drop most of tricks used before for that purpose.
2. Cached UTF-8 source code is used to improve performance
3. Improved error handling.
4. Slightly improved UI.

Change-Id: I4605200fa103167369a40650b2e1ad2c61e8133b
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/clangformat/clangformatplugin.h

@@ -30,7 +30,6 @@
 #include <memory>
 
 namespace ClangFormat {
-namespace Internal {
 
 class ClangFormatOptionsPage;
 
@@ -50,5 +49,4 @@ private:
     std::unique_ptr<ClangFormatOptionsPage> m_optionsPage;
 };
 
-} // namespace Internal
 } // namespace ClangTools