[KFileWidget] When saving, highlight filename after clicking existing file also when using double-click

Summary:
Currently, when using single-click mode, when you're saving a file and click on an
existing file, that file is highlighted and the filename field become focused so you
can quickly rename it. This patch makes that behavior work for double-click mode, too

Test Plan:
When using single-click:

• Save a file, single-click on an existing file in the file dialog
• Observe that current behavior has not changed
• Open a file, single-click on an existing file in the file dialog
• Observe that current behavior has not changed
• Save a file, double-click on an existing file in the file dialog
• Observe that current behavior has not changed

When using double-click:

• Save a file, single-click on an existing file in the file dialog
• Observe that the filename gets selected just like in single-click mode
• Open a file, single-click on an existing file in the file dialog
• Observe that current behavior has not changed
• Save a file, double-click on an existing file in the file dialog
• Observe that current behavior has not changed

Reviewers: Frameworks, VDG, broulik, cfeck, dfaure, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19849