Add fonts to QPlatformTheme.

Add fonts to QPlatformTheme.

- Remove QPlatformFontDatabase::defaultFonts() returning
  a hash containing widget name ->font and the Windows
  implementation.
- Add enumeration and font accessor to QPlatformTheme. The value
  returned for the enumeration value overwrites the default font
  of the font database.
- Implement for Windows, Mac and KDE.
- Add more Windows palettes.

Task-number: QTBUG-23686

Change-Id: I8a2abdfd216df23daa7c9630c54264cdf61295db
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>