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

Summary:
{nav File > Open Recent > Clear List} clears the displayed list of
recent files, but the same list in the {nav 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 {nav 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 {nav File > Open Recent > Forget All Files & Folders}
* Check if {nav File > Open Recent} is empty
* Check if {nav 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