Add style for readonly widgets

Some widgets support the readonly state, which currently doesn’t look any different from the default state. Taking ttk.Entry as an example, I’d consider either making it similar to the disabled state but with more contrast to keep the text readable, or give it the same style as a ttk.Label, with the border only appearing on hover and/or focus.

[israel-dryer]

I’m currently in the process of adding the state-based “readonly” style. As of now, I think using the light color for light themes and the selectbg color for dark themes seems to give the best result.

[daniilS]

In my specific use case, I use readonly entries as part of a table for cells that shouldn’t be editable. The difference from disabled entries is that the contrast makes them a lot less readable (which makes sense for the disabled style). Sticking with Bootstrap styles, I thought something like this could make sense?