Texteditor: Add ability to get margin from indenter

And implement it in the clang-format plugin to use the ColumnLimit from
the style. This would be quite useful if you're working on different
projects with a different ColumnLimit. This way you get an visual
representation and do not only have to rely on clang-format.

Change-Id: Ib0258e3fba6f45f0f46ce612f806527a47868ad9
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangformat/clangformatbaseindenter.h

@@ -66,6 +66,8 @@ public:
 
     bool isElectricCharacter(const QChar &ch) const override;
 
+    std::optional<int> margin() const override;
+
 protected:
     virtual clang::format::FormatStyle styleForFile() const;
     virtual bool formatCodeInsteadOfIndent() const { return false; }