Add fit page to window reader setting

[Alexandre-P-J]

Description

The following change adds a new option to the reader settings called fit page to window. This feature allows users to display pages maximally fitted to the reader window without overflowing vertical or horizontaly.
The behavior is similar to the current page fitting of the two-page or horizontal scroll reader modes. The reader style where the change makes more sense is in both single page modes but it is also useful in webtoon and continues vertical modes.

Before the change (identical to having the setting disabled)

Single page

Vertical

After the change: Disabled

Single page

Vertical

After the change: Enabled

Single page

Vertical

[schroda]

as you've mentioned in discord, I'd say the setting should be hidden for reader modes where it doesn't do anything

[Alexandre-P-J]

This last commit also hides the option in reader modes where it doesn't do anything.