Add style strategy to QFont to turn off automatic font resolution completely

The QFont::NoFontMerging flag turns off font merging within a single script item, but still allows automatically picking different fonts for each script item if the currently selected font does not support the script in question.

For some use cases, it would be useful to prohibit QFont from using glyphs from any other font than the one which is manually selected. Missing glyphs would then be represented as boxes to indicate to the user of the application that the currently selected font does not support the characters.