Add API for low-level access to font data

The lower levels of the text and font system in Qt are currently a black box. This can hinder communication with external libraries and application specifics when the application wants more control over text handling.

A requirement is to have access to the low-level font data through Qt. The main object would be to have a function in QPainter which allows drawing text based on a font, glyph indices and positions. This, in turn, requires accessors to get glyph indices and other font data based on a set of unicode points, or possibly more precise control over which font is selected for drawing the glyphs.

The actual requirements to make this API useful in as many use cases as possible will have to be investigated further.