Implement support for pregenerated glyph caches in Qt Quick

One of the major startup time bottle necks for UIs with a lot of text is rendering the glyphs into the cache for the first time. This is usually alleviated by designing the UI in such a way that the first screen is simple, without a lot of text.

To make the experience better for users, we could provide an opt-in system that allows pregenerating the glyph caches for certain fonts, allowing users to optimize the startup time manually.