Add missing setFocusProxy() call in ViewManager

Summary:
This should fix the terminal not gaining focus when using konsolepart.

BUG: 411181
FIXED-IN: 19.12.0

Test Plan:

• The terminal panels in Dolphin and Kate now always gain focus

properly, not just the first time you open them

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D25001