Fix View Mode session restore

Summary:
While restoring a saved session the last URL is selected but
View Mode is not triggered. This is caused by wrong order of
switching mode and setting the current URL.

BUG: 393176
FIXED-IN: 18.04.1

Test Plan:

• Open one or more instances of Gwenview in different states
• Logout with session restore enabled
• Login
• All Gwenview windows should restore in Browse or View Mode

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D12246