KateStatusBar: Show lock icon on modified label when in read-only mode

Summary:
This may not needed because the information is already shown as part of the
inputMode label, on the other hand is that information there somehow misplaced.

Test Plan:
You see the little lock bottom left?

Reviewers: KTextEditor, cullmann

Reviewed By: KTextEditor, cullmann

Subscribers: ngraham, cullmann, kwrite-devel, kde-frameworks-devel, KTextEditor

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D19512