Symbian^3 workaround: Avoid usage of linked fonts.

"Linked Fonts" are a new feature in Symbian^3, first used in SR11.1.
This patch prevents Qt from using linked fonts in any way.

Avkon based applications benefit from Linked Fonts because they can now
transparently render multi-script text, e.g. Latin/Chinese/Japanese (Qt
does that by its own and calls the feature 'font merging'). From Qt's
poing of view, Linked Fonts are a severe regression in Symbian's API.
1 Font table Api for linked fonts is extremely slow
2 'cmap' tables do not seem to reflect the "linkage", causing garbled
  text or crashes.
3 Linked fonts appear with cryptic type face names, and are redundant
  since they are just compounds of the other, real fonts.

This patch adds a detection of Linked Fonts using
CLinkedTypefaceSpecification and prevents their inclusion into
QFontDataBase. Furthermore, it detects if the Symbian System font is a
Linked font and if needed tries to fall back to a classical Symbian
system font, e.g. "Nokia Sans S60" or "Series 60 Sans". Fallback to
"Pure" will need to be added, later on.

Task-Number: QTBUG-20007
Co-authored-by: Miklos Vlasa
Reviewed-by: mread <qt-info@nokia.com>

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