Implement auto-completion when editing a space origin

[abey79]

#4848 introduced the ability to edit space origin for space views with a popup menu displaying suggestions. It would be nice to also have the ability to "tab-autocomplete" within the text edit.

This should be achievable as follows:

• use a custom Layouter for the text edit, where the autocomplete suggestion would be appended (with subdued styling) to the text actually entered by the user
• use lock_focus(true) to avoid defocusing the text edit with Tab (this entered a \t character instead)
• perform the autocomplete when \t is detected