TextEditor: make the format of completion item detail text adjustable

Instead of always assuming content in the form of rich text allow each
item individually to define it's text format for the detail text that is
shown as a tooltip of a completion item.

Fixes: QTCREATORBUG-22429
Change-Id: I9fa71373a743c26fa06d48acc5f0509584830ca0
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangcodemodel/clangpreprocessorassistproposalitem.cpp

@@ -131,6 +131,11 @@ QString ClangPreprocessorAssistProposalItem::detail() const
     return QString();
 }
 
+Qt::TextFormat ClangPreprocessorAssistProposalItem::detailFormat() const
+{
+    return Qt::RichText;
+}
+
 bool ClangPreprocessorAssistProposalItem::isSnippet() const
 {
     return false;