Enhancement: Add unstyled DockItem

[reyamir]

I wonder can I create a Dockitem without any pre styled?, I mean there are no Tab or Split styles, just empty DockItem, so I can add it to DockArea left_dock or right_dock then create custom sidebar.

Currently, I need to create the left panel like this:

let left_panels = DockItem::split_with_sizes(
    Axis::Vertical,
    vec![DockItem::tabs(vec![], None, &dock_area, cx)],
    vec![None, None],
    &dock_area,
    cx,
);

It will be more flexible if I can create custom left panel like this:

let left_panels = DockItem::custom(< some view >)

It will also help to solve #318 and #430 in the future

[huacnlee]

Give me some UI example for show up what do you want to do.

[reyamir]

I think your latest commit have fixed my issue, thank for great project
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