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

Summary:
For the open/save dialogs' {nav 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 {nav Open Containing Folder}. See that items are highlighted in the window that opens.
- Right-click on the background (i.e. select nothing) and select {nav 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