cppcheck-gui: add option to set tab width and font for code editor

[Zufu Liu]

tab width currently is the built-in hard-coded 8.
font is hard-coded "Monospace".

@@ -196,6 +196,7 @@ CodeEditor::CodeEditor(QWidget *parent) :
     font.setStyleHint(QFont::TypeWriter);
     setFont(font);
     mLineNumberArea->setFont(font);
+    setTabStopDistance(QFontMetricsF(font).horizontalAdvance(' ') * 4);