Part Revert "[Units] Make iconSizes property constant"

Summary:
Though we can make the property constant, some applications still use the
signal (for example FolderView.qml) to know that one of the icons has
changed sizes and do some processing.

Whilst it would be possible to fix folderview, we should avoid public
API changes.

Reviewers: Plasma, Frameworks

Subscribers: plasma-devel

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4092