Accessibility: Respect Preferred Font Size

Accessibility configuration options commonly include a setting for overriding the font size. The setting can either be a numerical DPI value, or a font size category (e.g. "larger fonts"). Qt should respect this setting in order to be usable for users who require a font size overrides.

Technical tasks:

• Determine which native font size settings exists on the various platforms, and how Qt should read/access the settings.
• Determine how the font size override should be applied to the Qt UI. Can we use a global DPI override, or should we introduce a new font size property?