Highlight selected items when showing parent folder from the open/save dialog

Summary:
For the open/save dialogs' Open File Manager context menu item, this patch adopts KIO::highlightInFileManager() and modernizes the name to match current convention ("Open Containing Folder").

BUG: 392330

Test Plan:
Deploy change and invoke the open dialog from Kate, then...

• Right-click on one or more items and select Open Containing Folder. See that items are highlighted in the window that opens.
• Right-click on the background (i.e. select nothing) and select Open Containing Folder. See that current folder is highlighted in the window that opens.

Reviewers: Frameworks, broulik

Reviewed By: broulik

Subscribers: broulik

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11705