Fix VI-Mode buffer commands

Summary:

• :tabclose currently makes kate crash.
• Switching tabs with commands like :bp or :tabp only works once due to loss of focus. The user must click into the view to reactivate it.
• Fix crash when switching documents with :b filename

Test Plan:

• use :tabclose to close a document
• switch documents several times with :bp, :tabn, etc.
• use :b filename to switch to corresponding tab

Reviewers: KTextEditor, cullmann

Reviewed By: KTextEditor, cullmann

Subscribers: cullmann, Frameworks

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D8710