Summary:
This patch adds an option to show the selected file(s), or the file currently being viewed in the file browser. It opens the default file browser and highlights the files.

{F5428928}
{F5428927}

Test Plan:
  - Select a file and right click on it, opening the contextual menu. Select the "Show in File Manager" option. The default file manager should open and highlight the file.
  - While viewing a file, right click on it, opening the contextual menu. Select the "Show in File Manager" option. The default file manager should open and highlight the file.

It also shows the option under the File menu.

Reviewers: ngraham, #kde_applications, rkflx

Reviewed By: ngraham, rkflx

Subscribers: rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D8282