Add way for turning off advanced shaping and kerning for text in Qt Quick

A pretty common use case when a text is updated at a high frequency, is that is is formatted as follows: <static prefix> [dynamic text consisting of numerals] <static postfix>.

Adding a way to
1. Anchor to the advance width of a text item

and

2. Tell Qt that OpenType features are not required for the numerals with an optimization flag

should improve performance in this use case significantly, since you would be able to put the static text in separate text items and skip a big part of the text layout process for the dynamic text.