Add TreeView

[microo8]

I couldn't wait for #113 and implemented the TreeView my self.
It can add/remove/setText of items.

usage is like this:

func main() {
	treeview := tview.NewTreeView()
	dbCur := treeview.AddItem(nil, "databases")
	treeview.ToggleItem(dbCur)
	treeview.AddItem(dbCur, "db1")
	db2Cur := treeview.AddItem(dbCur, "db2")
	treeview.ToggleItem(db2Cur)
	treeview.AddItem(db2Cur, "extensions")
	schemaCur := treeview.AddItem(db2Cur, "schemas")
	treeview.AddItem(db2Cur, "casts")
	treeview.ToggleItem(schemaCur)
	publicCur := treeview.AddItem(schemaCur, "public")
	treeview.AddItem(schemaCur, "schema1")
	treeview.AddItem(schemaCur, "schema2")
	treeview.ToggleItem(publicCur)
	tablesCur := treeview.AddItem(publicCur, "tables")
	treeview.AddItem(publicCur, "views")
	treeview.AddItem(publicCur, "types")
	treeview.AddItem(publicCur, "functions")
	treeview.ToggleItem(tablesCur)
	treeview.AddItem(tablesCur, "table1")
	treeview.SetItemText(treeview.AddItem(tablesCur, "table2"), "table20")
	treeview.RemoveItem(treeview.AddItem(tablesCur, "tableMeh"))
	if err := tview.NewApplication().SetRoot(treeview, true).Run(); err != nil {
		panic(err)
	}
}

And it looks like this:

I've tried to comment it as best as I could, I hope it will be OK.

[rivo]

Please see my comments to #113. I appreciate the effort but if I implemented a tree view, the API would probably look different. (But it probably won't be too much effort to replace your implementation with the "official" one at a later time.)