Remove all formatting API from `ListItem` and use `WidgetText`'s API instead

[abey79]

edit: thins have changed with list item 2, but this is still valid. There even is a todo about this in LabelContent.

ListItem accepts a WidgetText as input, so the formatting should happen there so that it doesn't the ListItem logic. Some helper might be needed to "prepare" the WidgetText though, e.g. to honour the re_ui::LabelStyle, etc.

[abey79]

One way forward would be to implement SyntaxHighlighting for SpaceViewBlueprint, Container, etc. as a mean to produce a impl WidgetText to feed to ListItem and other re_ui helper functions.