Add immutability to KCM Icons

Summary: Add immutability to KCM Icons, disable theme delegate and not the whole grid in order to allow user to change icon size if not immutable too

Test Plan: Add [$i] to kdeglobals key

Reviewers: #plasma, ervin, ngraham, crossi, meven

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26045