Add 'Open' event on modal component

[mat02]

Proposed changes

Fixes #978 #1104 by adding event handler to Modal component, so actions can be performed after Modal dialog is shown.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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Further comments

This is pretty minor change in the code. As this is my first contribution directly to the codebase I am not sure if this requires some additional changes. I've used antd 'afterOpenChange' event handler to implement this, which has a sideeffect that it only starts after the modal is rendered, but I think this is not an issue in most cases.

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[ludomikula]

Thank you!