Expose "Toggle Comment" feature through tools menu and default shortcut

Summary:
Adds a menu item to toggle comments and adds CTRL + / as a shortcut. Also removes "Comment"/"Uncomment" menu items (but keeps the shortcuts to avoid breaking muscle memory).

BUG: 387654

Test Plan: Test both old and new shortcuts and the new menu item.

Reviewers: VDG, ngraham, Kate, KTextEditor, cullmann

Reviewed By: VDG, ngraham, Kate, KTextEditor, cullmann

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

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17008