https://gjs.guide/extensions/upgrading/gnome-shell-46.html#clutter-container

This adds a new helper function 'getZoomModeEnum()' to provide a list of valid zoom mode values from the new 'getEnumFromSettings()' which retrieves all possible enum values from a given gsettings schema.
Fix #192

Co-authored-by: Wolfgang Rumpler <[email protected]>

This change is necessary for updating wallpapers to work on clean
installs. The number 2 now maps to zoom.

Unfortunately, the number has no direct relation to the valid values and
changing the filter on the available picture-options will cause a
regression here.

This is solely done for user feedback when changing the value.

The recommended "add_child" call doesn't behave exactly the same as
"add_actor" in 45 and causes the history section to render incorrectly.
With this commit, the working function is used depending on the major
version.

* Change the name to "Scaling Mode" to be more generic
* Bind settings UI directly to the gnome shell background and
  screen-saver settings to provide direct feedback when changing the
  value
* The internal copy of the currently selected mode is kept as string to
  restore the selected value when not using an external wallpaper
  manager with multi-monitor support