Summary:
In previous versions of Gwenview it was possible to toggle between
{nav Fit} and {nav 100%} zoom factors using the shortcut {key F}.
After 3e10699a added the {nav Fit Width} zoom option this feature
did not work any more.
This patch adds toggling for both {nav Fit} and {nav Fill} zoom
options if the shortcut {key F} or one of the buttons is pressed
multiple times. Zooming to 100% is centered to the current cursor
position like middle click works.

BUG: 396360

Test Plan:
# Open Gwenview in View Mode
# Use zoom buttons {nav Fit}/{nav Fill} or {key F} multiple times
# The zoom should toggle between {nav Fit} and {nav 100%},
  respectively between {nav Fill} and {nav 100%}

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D14093