Fix double delete bug in OutputWidget

Summary:
The bug was unfortunately introduced in D14931 by me - two shared pointers
were created from the same raw pointer. Eliminated it by using QSharedPointer
as a return value from view creation helper lambda.

Test Plan:
The bug can be triggered only in IOutputView:OneView widget mode, which is (as
far as I can see) currently neither used, nor properly tested. Planning to change
this in following patches.

Reviewers: kfunk, kossebau

Reviewed By: kfunk

Subscribers: anthonyfieroni, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D15241