Expose KTextEditor::ViewPrivate:setInputMode(InputMode) to KTextEditor::View

Summary:
This makes possible to change the View Input Mode using KTextEditor::View:setInputMode(InputMode).
This is useful for when using *KTextEditor* as an external library in an another project. The developer should be able to change the input mode programmatically.

Reviewers: kossebau, mlaurent, vkrause, dhaumann, demsking

Reviewed By: dhaumann

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

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D16347