Extend action "Clear Recent List" to clear model and recent folders

Summary:
File → Open Recent → Clear List clears the displayed list of
recent files, but the same list in the Start Page → Recent Files
tab is not cleared (only after restarting Gwenview).
This patch extends and renames the menu entry to clear the internal
models and both tabs Start Page → Recent Files / Folders.

Ref T8194
FIXED-IN: 18.04.0

Test Plan:

• Open some images to populate the recent files/folders lists
• Click File → Open Recent → Forget All Files & Folders
• Check if File → Open Recent is empty
• Check if Start Page → Recent Files / Folders are empty

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Maniphest Tasks: T8194

Differential Revision: https://phabricator.kde.org/D11477