Implement add panel to root for DockArea

[reyamir]

I've tried to implemented fn add_panel_to_root for DockArea, but the view is not reload after add panel. Do you know what I'm missing?

// DockArea
...
pub fn add_panel_to_root(&self, panel: Arc<dyn PanelView>, cx: &mut ViewContext<Self>) {
    if let DockItem::Tabs {
        items,
        active_ix,
        view,
    } = &self.items
    {
        view.update(cx, |view, cx| {
            view.add_panel(panel, cx);
            cx.notify();
        })
    }
}
...

[reyamir]

I've updated my fn into this

pub fn add_panel_to_tabs(&self, panel: Arc<dyn PanelView>, cx: &mut ViewContext<Self>) {
    match self.items.clone() {
        DockItem::Tabs {
            mut items,
            mut active_ix,
            view,
        } => view.update(cx, |view, cx| {
            view.add_panel(panel.clone(), cx);
            items.push(panel);
            active_ix += 1;
            cx.notify();
        }),
        DockItem::Split { view, items, .. } => view.update(cx, |_, cx| {
            for item in items.into_iter() {
                if let DockItem::Tabs {
                    mut items,
                    mut active_ix,
                    view,
                } = item
                {
                    view.update(cx, |view, cx| {
                        view.add_panel(panel.clone(), cx);
                        items.push(panel.clone());
                        active_ix += 1;
                        cx.notify();
                    })
                }
            }
        }),
    }
}

and it worked now

[huacnlee]

Do you still need my help? It looks like it's ready. I'm also going to open a branch to check the problem.

[reyamir]

Ah, it seems work fine now. I will create a PR if you like, I think this fn is useful for #318 , user can create a sidebar then add new tab when user click to sidebar's component

[reyamir]

Other things I wonder is do I need to push new panel to items vec here:

view.add_panel(panel, cx);
items.push(panel); <-

or just view.add_panel is enought for add new tab