The method horizontalAdvance() replaces width() starting with Qt 5.11.
To handle this, all direct calls to QFontMetrics::width() are replaced
by calls to GuiFontMetrics::width(), and the code for
GuiFontMetrics::width(QChar) uses horizontalAdvance on newer Qt
versions.
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.
