macos: Fix popup window and keyboard input

This avoids the focus to move to a popup or popover window.

The root (toplevel) window is set to retrieve the key events, therefore popups do not need to be a "key window" anymore.

Fixes #6950 (closed).

Fixes #5049 (closed): the red/yellow/green buttons remain colored.

• Do we really not need popups to be key windows? Are there cases where a popup window is actually the root window (that are not programming errors 😉 )?
• Ensure the interplay between IMContext and GdkMacosbaseView works (gobject data)
• Position Input Source popup below text in popups.