Place cursor after opening brace for function implementation

Summary:
This will place the cursor after the opening brace for the generated
body of the function implementation, similar to how the cursor is placed
after the opening parenthesis of an executed function declaration
completion.

Reviewers: brauch

Reviewed By: brauch

Subscribers: brauch, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D16386