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
In particular, the directory frontends/qt4 is renamed to frontends/qt.
Many configurations file have to be updated. All mentions of qt4 in
the source have been audited, and changed to qt if necessary.
The only part that has not been updated is the CMake build system.
