Do not use a static variable as QTextLayout cache

Do not use a static variable as QTextLayout cache

It is a bad idea to have a QObject variable that oulives the main QApplication object. See for example:
https://www.ics.com/designpatterns/book/globals.html

Here the QTextLayout object was static to the anonymous namespace getTextLayout function, and got destroyed after the freetype renderer had been disposed of by QApplication.

This causes segmentation faults when quitting LyX on some systems.

This patch moves the cache together with other GuiFontMetrics caches. It means that one will have one such QTextLayout per font type, but this will not change much.