Disabled widgets editable using keyboard

[slook]

You can edit the text after tabbing focus into it using keyboard navigation if the previous widget has FocusPolicy.ClickFocus Edit: or maybe because the previous widget is also itself disabled.

Ideally the focus should skip disabled widgets and focus the next widget that isn't disabled (if any), or otherwise clear the focus to prevent typing into disabled widgets.

Might be a regression since #537

Edit: It is also possible to tab focus into a disabled TTkButton and press the Return key on it.

Sometimes the incorrect widget is activated on press, when clicking on a widget where a FocusPolicy is set then some other widget (I think either the previously focused one or next in focus chain) gets the click event instead.

Update: I'm seem to be having several cases where widgets are completely invisible upon creating a new window overlay until they are focused for the first time, that might be related to the same regression or something, but I haven't figured out why yet.