Pass some layout props to panel's content container

[cnrudd]

Toolbox branch: https://github.com/xh/toolbox/tree/paddingOnPanel
Toolbox PR: xh/toolbox#415

This PR resolves issue #1971 . I found that in addition to 'padding' it made sense to pass the following style keys to the content container:
'overflow', 'alignItems', 'alignContent', 'justifyContent'

If others don't share my enthusiasm for all of these, I could back down on 'alignItems', 'alignContent', 'justifyContent', but 'padding' and 'overflow' are great.

I was pleasantly surprised by how many cleanups were possible in Toolbox (see PR) by using this change.

The one disadvantage to this is that the app's standard padding value --xh-pad-px is not available to the JS when using panel({padding: 10}). This could be resolved by creating a js version of the value on the appModel or some other central location, but leaving that for discussion.

Another solution could be creating a prop called contentClassname that takes a css class(s) and passes it on to the content container. This would save the dev from having to always write .panelClass > .xh-vframe { ... to style the content.

Also wondering if it is worth documenting on Panel.js what style props are accepted and placed where. 'width', 'height', 'flex', etc... have all been accepted and working but not documented clearly. Now with some style props being applied differently, it might be worth putting in the docs which apply where.

Hoist P/R Checklist

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• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes: CHANGES SO FAR ARE NOT BREAKING CHANGES
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required.
• Created Toolbox branch / PR, or determined not required.

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