Implement robust minimum pane size

[tim-blackbird]

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We need a robust pane resizing implementation that prevents panes from being resized below a minimum size.

The minimum size along the height should be the height of the panel header. I'm don't see an obvious choice for the minimum width. I'd pick the panel header height as minimum here as well for now.

Keep in mind that this also needs to prevent the panes going below the limit when the entire window is being resized.