Deprecate the global [Small|Desktop|Bar]Icon() methods

Summary:
They are not namespaced and promote the use of pixmaps rather than
icons. Additionally their API allowing to specify a size that differs
from the size suggested by their name leads to bizarre usage like
'SmallIcon("foo", KIconLoader::SizeLarge)'.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: dfaure, ngraham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11654

Differential Revision: https://phabricator.kde.org/D24136