Dehardcode script settings window font and colors

A Lua-based scripts setting menu always uses a font height 16 and fixed text colour. It does not take an account a font size setting from settings.cfg and text colour settings from openmw.cfg

A suggested approach to fix this issue:

1. Add a Lua function to get a font size setting's value.
2. Since raw text values from openmw.cfg are already available via core.getGMST, add a helper function to parse these values as RGBA colours.
3. Once new API from steps 1 and 2 is implemented, use it in the constants.lua.