Replace QFontMetrics::width() by horizontalAdvance() in Qt>=5.11

Replace QFontMetrics::width() by horizontalAdvance() in Qt>=5.11

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.

(cherry picked from commit 21422dd6527754d8b7876a6e11b686cd726321ed)