Feature Request: add containers for widgets

The way to set the positions of widgets is through the x1 y1 coordinates in func (*Block) [SetRect](https://github.com/gizak/termui/blob/v3.1.0/v3/block.go#L92):
```go
p := widgets.NewParagraph()
p.Title = "Lunar"
p.Text = selectedAudio
p.SetRect(0, 0, 40, 3)
p.TitleStyle.Fg = ui.ColorYellow
p.BorderStyle.Fg = ui.ColorCyan

c := widgets.NewParagraph()
c.Title = "Audio Controls"
c.Text = `Pause and play music: [ENTER]
Volume: [↓ ↑]
Speed: [← →]
Normal Speed: [Ctrl + N]
Back to menu: [BACKSPACE]
Quit Lunar: [ESC]
`
c.SetRect(0, 4, 40, 12)
c.TitleStyle.Fg = ui.ColorYellow
c.BorderStyle.Fg = ui.ColorCyan

ui.Render(p, c)

```
But this can create overlap over widgets:
```power shell
+-Lunar--------------------------------+
|Playing Charlie Puth |
+--------------------------------------+
|Normal Speed: [Ctrl + N] |
|Back to menu: [BACKSPACE] |
|Quit Lunar: [ESC] |
+--------------------------------------+
```
Preferred output:
```powershell
+-Lunar--------------------------------+
|Playing Charlie Puth |
+--------------------------------------+

+-Audio Controls-----------------------+
|Pause and play music: [ENTER] |
|Volume: [↓ ↑] |
|Speed: [← →] |
|Normal Speed: [Ctrl + N] |
|Back to menu: [BACKSPACE] |
|Quit Lunar: [ESC] |
+--------------------------------------+
```

My idea is to create a container for the widgets like in the [Fyne GUI](https://developer.fyne.io/container/box) to position
the widgets and avoid widget overlap:

```
SetContent(NewVericalBox(c, p))
```

Start with v3/block.go at Block.SetRect and the rendering path used by ui.Render, then inspect how widgets.NewParagraph currently receive and retain positions. Compare the requested container behavior with the existing widget APIs and determine how layout should be specified. Done means a container can position multiple widgets without overlap, with behavior demonstrated by suitable tests or examples.

Written by the indexing model from the issue text.