Add Zoom Controls widgets

[wpkelso]

Problem

Zoom buttons and scales are used in a fair amount of apps. The buttons (or the idea styled the same) are used in at least Web, Code, Files, and -- in the community -- Jorts. The scale is less used, but it is still present in Photos, and probably some other community apps. Therefore, I think it's reasonable to provide a widget built into the library, so that all uses will have common accelerators, styling, etc., and we don't have to repeat ourselves so many times across apps.

Proposal

Add two widgets:

• Zoom buttons
• Zoom scale

Prior Art (Optional)

[teamcons]

You can freely grab from ZoomBox if you want: https://github.com/ellie-commons/jorts/blob/main/src/Widgets/PopoverWidgets/ZoomBox.vala