Add QWinMime.

Add class QWinMime (equivalent to WindowsMime in Qt 4).

[ChangeLog][QWinMime] Added abstract class QWinMime (equivalent
to WindowsMime in Qt 4) for registering custom mime type
conversions.

Task-number: QTBUG-39559
Change-Id: Ie8ff4db6cd0ce64f65b83232dc91d771238663d1
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>