KFilePlaceEditDialog: move logic into isIconEditable()

Summary:
Instead of relying on whether m_iconButton has been initialized
somewhere else in the code.

Test Plan: Make sure that editing icons is still allowed only for non-Trash places.

Reviewers: Frameworks

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26277