fixes #3654 - NumericUpDown not focusable by pressing tab

[corvinsz]

set IsTabStop to False on the default NumericUpDown control style

[corvinsz]

removed setting IsTabStop again. Instead added an eventhandler for GotFocusEvent to move the focus to the PART_TextBox when the NumericUpDown has gotten focus.

When the control gets focused, all the text of the PART_TextBox gets selected via SelectAll(). It's debatable if that should be a thing or not. I decided to go with it because that's how the default WPF DatePicker handles it.

[Keboo]

Hey @corvinsz, while reviewing your PR, I'd suggest the following code changes:

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https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/pull/3656

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[github-actions[bot]]

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