Zoom affordance on FXL

[HadrienGardeur]

While FXL defaults to having both dimensions of the page and/or spread contained in the viewport, that's not always the optimal or preferred way for the reader to handle it.

In addition to pinch/zoom, we want to also support a zoom/fit affordance with the following values:

• Fit page (default)
• Fit width
• Fit height
• and the ability to use +/- with a fixed stepper to zoom in/out (zooming completely out will default back to fit page)

While this is currently listed in #1, it might be pushed out to later in the development cycle as it would usually rely on the Preferences API as well.