KFileWidget: Disable zoom buttons once reached minimum or maximum

Summary:
In Gwenview the zoom buttons will be disabled when clicking on it would
not result in any further change of zoom level, e.g. Zoom In is
disabled for the largest icon size. In general it is recommended to
disable unavailable actions.

This patch adds the functionality to the file dialog too.

Ref T8552

FIXED-IN: 5.46

Test Plan:
kdialog --getopenfilename, change icon size to minimum and maximum by
sliding, mouse wheeling, pressing buttons and using the keyboard, and
check that buttons are disabling and enabling properly.

Reviewers: Frameworks, ngraham

Reviewed By: ngraham

Subscribers: ngraham

Tags: Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D12590