Fix Qt deprecation warns for setTextColor()

Fix Qt deprecation warns for setTextColor()

This commit fixes the following warning from Qt 5.14:

  error: ‘void QTreeWidgetItem::setTextColor(int, const QColor&)’ is deprecated: Use QTreeWidgetItem::setForeground() instead [-Werror=deprecated-declarations]

setForeground was introduced in Qt 4.2 [1].

We have a couple of commented out uses of a setTextColor() method in
GuiDocument.cpp, but the QTextEdit::setTextColor() is not deprecated so no
change is needed.

[1] https://doc.qt.io/archives/qt-4.8/qtreewidgetitem.html#setForeground