[GB.GTK]
* NEW: Rework Design property management so that it works better.
* OPT: Cache font metrics to speed up some font properties.

[GB.GTK3]
* NEW: Rework Design property management so that it works better.
* OPT: Cache font metrics to speed up some font properties.

[GB.QT4]
* NEW: Rework Design property management so that it works better.

[GB.QT5]
* NEW: Rework Design property management so that it works better.