Clang: Add tooltip action to copy to clipboard

...as selecting text in the tooltip was difficult and eventually got
disabled due to other problems - see
d58c0a9ac8.

This adds support for actions in TextMarks. They are displayed as
QToolButtons in a dedicated column in the tooltip.

Change-Id: I84ee3c3e4af573a80953786881d1333b00e4200c
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdiagnostictooltipwidget.cpp

@@ -474,9 +474,17 @@ QString ClangDiagnosticWidget::createText(
     const ClangDiagnosticWidget::Destination &destination)
 {
     const QString htmlText = WidgetFromDiagnostics(toHints(destination)).htmlText(diagnostics);
+
     QTextDocument document;
     document.setHtml(htmlText);
-    return document.toPlainText();
+    QString text = document.toPlainText();
+
+    if (text.startsWith('\n'))
+        text = text.mid(1);
+    if (text.endsWith('\n'))
+        text.chop(1);
+
+    return text;
 }
 
 QWidget *ClangDiagnosticWidget::createWidget(