Add more font quality options for Static font engine

Currently it is possible to set font quality either to Font.QualityVeryHigh or to Font.QualityVeryLow when optimizing binary size. This is important optimization feature in Static font engine especially for internationalized Qt for MCUs applications.

Unfortunately, visually glyphs with Font.QualityVeryLow are not very pleasing but have considerably smaller memory footprint.

As a suggestion, adding more options (for example Font.QualityAverage) would perhaps offer a better tradeoff between visual quality and memory footprint on flash. The way how current font quality options are named gently hint that there should be something in between those existing two options.