Refactor painting of editor gutter

pglab/codeeditor/GutterPainter.h

@@ -0,0 +1,38 @@
+#include <QPainter>
+#include <QTextBlock>
+
+#pragma once
+
+class CodeEditor;
+class QPaintEvent;
+
+class GutterPainter
+{
+public:
+    GutterPainter(CodeEditor *editor, QPaintEvent *event);
+
+    void Paint();
+private:
+    CodeEditor *editor;
+    QPainter painter;
+    QPaintEvent *event;
+    QFontMetrics fontMetrics;
+
+    // Loop state
+    struct LoopState {
+        CodeEditor *editor;
+        QTextBlock block;
+        int top;
+        int bottom;
+
+        LoopState(CodeEditor *editor);
+
+        int blockNumber() const;
+
+        void advanceToNextLine();
+    };
+
+    void drawLineNumber(const LoopState &loopState);
+
+    void drawGutterErrorMarker(const LoopState &loopState);
+};