Support for Symbian Font linking in Qt Font system

Qt has its own mechanism for font merging. But on S60 side, it is needed to control how the characters/glyphs are
retrieved from the Qt font system.

In Qt, Qfontdatabase_s60 is hardcoded to only load the .ttf and .ccc files. The linked font files with .ltt extension are not loaded. Hence, it is necessary to update the Qt port for S60 code to load the .ltt files as well.

This is needed in order to ensure that correct font is retrieved from the correct font files so that text renders properly within the given layout. Another use case would also be to ensure for example in Chinese variants, the Latin characters should come from the Latin font file and not the Chinese font file.