Adapt for GNOME 50

[ulugbekeshnazarov42]

This updates the extension for GNOME 50 compatibility.
Changes:

• Remove legacy global.screen fallbacks and use global.workspace_manager / global.display.
• Replace global.screen_height with global.stage.height.
• Fix the all-workspaces switcher filter so it includes eligible windows.
• Preserve the default GNOME Shell switcher handler and restore it when the extension is disabled.
  Tested:
• make schema passes.
• git diff --check passes.
  Note:
• make build could not be completed locally because msgfmt was missing from the system.

[dsheeler]

@ulugbekeshnazarov42, thank you so much for the pull request! I looked it over and it looks good, and I could merge everything except the two things on which I commented: the change to manager.js and the version number in metadata.json. Could you fix up those things?

Great to have a new contributor, thanks!