MainWindow: Add indicator for read-only files

Summary:
This fixes the Bug 134854
If current file is read-only (checked with ReadWritePart::isReadWrite()) the window title changes - "(read only)" is added.
To check if the read-only status changed KTextEditor::Document::readWriteChanged signal is used.
Note: chmod-ing a file won't make in read only for ReadWritePart::isReadWrite().

Test Plan: Seams to work fine.

BUG: 134854

Reviewers: kfunk

Reviewed By: kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D794