Widgets: Add TerminalView

[wpkelso]

Introduces a TerminalView widget that

• Automatically includes terminal styling from Styles: Add Granite.CssClass.TERMINAL #864 , which is now an internal style class
• Includes toggleable autoscroll functionality
• Uses a scrollable TextView

Also:

• Switches message dialog to use this new widget
• Moves Terminal View to it's own page in the demo app

Future extensions

• Include option to automatically tie to stdout or stderr
• Support ANSI graphics and color escape codes

[stsdc]

If it's possible with this widget, maybe it would be worth to show colour palette?

Small snippets on how to archive those colours. e.g. for [PASS], [FAIL], [WARNING].

[wpkelso]

If it's possible with this widget, maybe it would be worth to show colour palette?

Supporting ANSI Graphics/Color escape codes would be useful. However, I don't know whether it would be better to delay this PR to add that support, or to include that support in a follow-up.

—Edit:
Thinking further on it, this is probably better in a follow-up. At minimum (thinking out loud), I expect this would require a fully defined TextTagTable, some reworking of the stylesheet to allow us to actually apply foreground colors, and some mechanism to apply the tags.

[danirabbit]

I still don't think this should be a public widget. The previous style class was really only used in MessageDialogs, it wasn't supposed to be a style that app developers were using regularly. So personally I'm -1 on this altogether. This isn't something I think we should be doing or supporting.

Tbh even in MessageDialogs it's been a bit of an antipattern. People have been really bad about not reading these messages, even when reporting an issue they won't expand the terminal view. So I'm kind of in favor of removing this feature altogether in a future version of MessageDialog and forcing developers to write some better error handling

[danirabbit]

This needs documentation. See some of the other widgets for an example. Just needs a simple description of what this widget is and what it's used for

[danirabbit]

I don't think this should be added as a constant here. We usually don't make constants for things that aren't used outside of their own class

[danirabbit]

Instead of adding a style class, this should be the css name. Right now you have css nodes like widget.terminal but it should just be terminal So you need to remove this css class and add something like:

    class construct {
        set_css_name ("terminal-view");
    }