Add note about porting loadMimeTypeIcon

Summary: We're phasing out loadMimeTypeIcon in favor of QIcon::fromTheme(). We can't oficially deprecate it yet since there are still unported users, but add a comment recommending alternative API.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28831