Windows: Improve debug output of mime conversion handling.

Introduce convenience function for obtaining the format
name and use that.
Add debug operator for struct FORMATETC including format name.
Replace all code enclosed in #ifdef QMIME_DEBUG by qCDebug().

Task-number: QTBUG-47656
Change-Id: Ibb3c860c57edee4a31e8f5f37e3245feebed52a1
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>