Introduce MimeTypeChecker::hasWantedMimeTypes

This allows us to skip the slow MimeTypeChecker::wantedMimeTypes
call, which allocates a QStringList on every call. This removes
~200k temporary allocations that I see in one heaptrack file for
kmail.

Reviewed By: dvratil
Differential Revision: https://phabricator.kde.org/D7916