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/clangformatconfigwidget.h

@@ -32,7 +32,6 @@
 namespace ProjectExplorer { class Project; }
 
 namespace ClangFormat {
-namespace Internal {
 
 namespace Ui {
 class ClangFormatConfigWidget;
@@ -49,9 +48,11 @@ public:
     void apply();
 
 private:
+    void initialize();
+    void fillTable(const std::string &testFilePath);
+
     ProjectExplorer::Project *m_project;
     std::unique_ptr<Ui::ClangFormatConfigWidget> m_ui;
 };
 
-} // namespace Internal
 } // namespace ClangFormat