Add shortcut Shift+F for zoom option "Fill"

Summary:
This patch adds the shortcut {key Shift F} for zoom option {nav Fill}.

CCBUG: 396360

Test Plan:
# Open Gwenview in View Mode
# Use {key Shift F} to switch to {nav Fill} zoom factor

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14094