Replace "Fit Width" feature with "Fill"

Summary:
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.

FEATURE: 195579

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

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D10844