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/texteditor/texteditor.cpp

@@ -2474,6 +2474,7 @@ void TextEditorWidget::keyPressEvent(QKeyEvent *e)
         e->accept();
 
         if (extraBlocks > 0) {
+            cursor.joinPreviousEditBlock();
             const int cursorPosition = cursor.position();
             QTextCursor ensureVisible = cursor;
             while (extraBlocks > 0) {
@@ -2493,6 +2494,7 @@ void TextEditorWidget::keyPressEvent(QKeyEvent *e)
             }
             setTextCursor(ensureVisible);
             cursor.setPosition(cursorPosition);
+            cursor.endEditBlock();
         }
 
         setTextCursor(cursor);