keyboard: allow going back from CCInputChooser via single click

When adding new input sources, the first dialog shows a list of languages, when single clicking one item you go to a subdialog that has a 'go back' item, that item only works if double-clicking it, but that is unintuitive because if you entered the dialog by single click you also expect the 'go back' button to work by single click too.

The subdialog is a GtkListBox configured to be double click, because it has an 'Accept' header bar button and we don't really want to activate the subdialog items by single click, but the 'go back' row is special and as explained before it is expected to work by single click too.

So this commit adds the required single click handler for the 'go back' rows created in CCInputChooser.

Fixes #1954 (closed)