Expose KTextEditor::MainWindow::showPluginConfigPage()

Summary:
The application already has a counterpart that takes a (Plugin*, uint),
so we need to adapt this to (Plugin*, int).

Test Plan: make && make test

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D24164