KPropertiesDialog: switch to label in setFileNameReadOnly(true)

Summary:
for consistency, when IconApplet calls this method it shouldn't use
a readonly lineedit, but a QLabel.

Test Plan:
locally hacked tests/kpropertiesdialogtest.cpp to add a call to
setFileNameReadOnly(true)

Reviewers: rkflx, shubham, ngraham

Reviewed By: rkflx

Subscribers: bruns, michaelh, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14662