Abstract the re_ui::ListItem code for re_viewer_context::Item

[abey79]

In a growing number of place, we are now using re_ui::ListItem to display and interact with re_viewer_context::Items. That includes:

• the blueprint tree
• the streams tree
• the top Selection Panel banner

In the future, this will also include:

• the container children in the selection panel (when a container is selected, part of New container/space view addition workflow #4492)
• the space view queries (when/if we support multiple queries per space views)
• the component list when entities are selected (Use the ListItem style for the component list (in the Selection Panel when an entity is selected) #4161)
• others?

Currently, the ListItem code is duplicated, along with the logic around selection highlight, selection update on click, shared hover state, etc. Ideally, the UI code for this would be unified and reused across the viewer.

To be addressed

• There is significant overlap with the existing "Data UI" code, in particular impl DataUi for re_viewer_context::Item. However, it's not clear that both the "ItemListItem" and the "Data UI" code can be fully merged, as they serve slightly different (albeit sometime overlapping) needs.
• A given type of item might need slightly different behaviour depending on context. For example, a Space View listed in the Blueprint tree might have a "visibility toggle" button, but not when listed as a child from a container in the Selection Panel.
• Also, sometime items are to be displayed as part of a hierarchy, and other times as part of a flat list.

Possible pragmatic approach

• Accept these are two different things and roll a new ListUi trait along DataUi, in the same re_data_ui crate.
• Somehow make it flexible enough for the contextual variations.

[abey79]

Relevant FigJam mapping out all existing and short-term-future (potential) use of ListItem: https://www.figma.com/file/6IiI4qAPhA6T8fvb8QuzJ2/ListItem-2.0-deployment?type=whiteboard&node-id=0%3A1&t=5s8vLxlXmqDYNp7A-1