Use QActionGroup for input mode selection

Summary:

• now shows the current mode at start
• automatically is mutually exclusive

Test Plan: - select either normal or VI mode

Reviewers: KTextEditor, dhaumann

Reviewed By: KTextEditor, dhaumann

Subscribers: dhaumann, Frameworks

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D8147