Fix context menus for Recent Files / Folders on Start Page

Summary:
On {nav Start Page > Recent Files} the context menu is missing and
right clicking on an item switches to View Mode.
This patch adds the context menu to {nav Recent Files} tab items
with the entries {nav Add Containing Folder to Places},
{nav Forget This File} and {nav Forget All Files}. The entries on
the {nav Recent Folders} tab are renamed accordingly.

Ref T8194
FIXED-IN: 18.04.0

Test Plan:
* Open some images to populate the recent files/folders lists
* Go to {nav Start Page}
* Right click on file / folder items to show context menu
* Check actions to execute correctly
* Check if clear actions for recent files sync with {nav File > Open Recent}

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Maniphest Tasks: T8194

Differential Revision: https://phabricator.kde.org/D11479