Fix context menus for Recent Files / Folders on Start Page

Summary:
On 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 Recent Files tab items
with the entries Add Containing Folder to Places,
Forget This File and Forget All Files. The entries on
the 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 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 File → Open Recent

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Maniphest Tasks: T8194

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