`ListItem` 2.0 (part 3): `PropertyContent` column auto-sizing

This PR:

Implements column auto-sizing for PropertyContent. The current heuristics is rather simple and will likely need improvement when it's battle tested. Currently, the content requests a minimum width of 200px (there really isn't a way to properly deal with two-column-plus-button neatly all the way to 0 width), and allocate a maximum of 70% of the available width to the label column.
Improves the way ListItem handles content capturing the hover state (which happen for interactive content). There is no longer a requirement for ListItemContent::ui() to return an Option<Response>, which simplifies a bunch of things.
Updates re_ui_example accordingly

Note: still no change applied to the actual viewer code base so far.

Improved heuristics for column sizing, when we have a better idea of the need.
More helpers are needed for various kinds of values.
There can be only 0 or 1 action button. This should be extended by using a … button with some kind of popup with all available actions in a future PR.
Right gutter space is reserved for the action button even if no list item in scope use them. The list_item_scope could track this and skip reserving that space if it's never used (e.g. component list in entity path selection panel): https://github.com/rerun-io/rerun/issues/6179

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- Using examples from latest main build: rerun.io/viewer
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rerun-io / rerun