Accessibility: keyboard navigation bug in profile editor

[vvillenave]

Steps to reproduce the behaviour

Open the Profile editor (Alt-E, Up, Enter) and go to the “Background” tab
(e.g. by hitting Ctrl-PageDown three times).

The Tab title should be focused; hit Tab once to access the radio buttons
contained in that tab.

Expected & actual behaviour

When “Solid color” is selected, hitting Tab once more should bring you to
the following interactable widget, right? Wrong: it actually brings you to
the Image FileChooser button, which is part of the second radio button
(“Background image”). There’s no visual nor accessible indication that
this is actually where the focus is; furthermore, that button is not interactable
with the keyboard only: neither Return nor Space will actually open the
file chooser dialog. (Clicking on it works, though -- although it shouldn’t
if that widget were properly disabled when the corresponding radio button
isn’t active.)

If you decide that you actually do want a background image, hit Shift-Tab once
to go back to the radio buttons, and hit Down to select the “Background image”
option. Now the FileChooser button should be enabled, right? Wrong: hit Tab
once more, then Space: again, nothing happens.

I suspected hidden widgets such as “Scroll background” might be in play,
but it turns out not to be the case. Reordering the GtkBuilder XML spec doesn’t
appear to help either.

MATE general version

Reproduced with both 1.20 and master. (Actually, when trying to bisect this
I haven’t found any version where it worked correctly.)