Minor code clean-up

Change-Id: I5c43db4e0a499ecfd23f3dac63615e7c40208f36
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Rename some QTextEngine members to reflect their meaning

formats() -> formatCollection()
additionalFormats -> formats

QTextEngine has three different code paths: in context of QTextDocument,
additionalFormats are currently _additional_ formats;
though in QTextLayout, they are the only formats in use;
and the QRawFont-related path shares the QTextLayout's behavior.

This is a preparation step to consolidating these three into a single one.

Change-Id: I427ccc3c2f672ce090899bb0a0995972315daafa
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>