Replace "Fit Width" feature with "Fill"
Fit width is arguably less useful than a more general Fill feature. This replaces "Fit Width" with "Fill", fitting width or height, depending on what fills the window.
Test Plan: Use Fill with landscape and portrait pictures with the window in landscape and portrait size.
Reviewers: Gwenview, ngraham, rkflx
Reviewed By: Gwenview, ngraham, rkflx
Subscribers: rkflx, muhlenpfordt, ngraham, Gwenview
Differential Revision: https://phabricator.kde.org/D10844