Add tree-of-models bubble component

[wagoodman]

This adds a new component to render a tree of generic tea.Models:

 ✔ a
   ├── ✔ a-a
   ├── ✔ a-b
   │     ├── ✔ a-b-a
   │     ├── ✔ a-b-b
   │     └── ✔ a-b-c
   ├── ⠼ a-c
   │     └── ⠼ a-c-a
   └── ⠼ a-d
 ⠼ b
   ├── ⠼ b-a
   │     ├── ⠼ b-a-a
   │     │     └── ⠼ b-a-a-a
   │     │           └── ⠼ b-a-a-a-a
   │     └── ⠼ b-a-b
   └── ⠼ b-b

This will ultimately drive a future version of syft's ui output:

 ✔ Parsed image
 ⠹ Cataloging contents             ━━━━━━━━━━━━━━━━━━━━    
   ├── ⠹ Packages                        [183 packages]
   │     └── ⠹ Java archive cataloger
   ├── ✔ File digests                    [14,771 digests]
   └── ✔ File metadata                   [16,428 locations]

This adds the concept of a VisibleModel:

type VisibleModel interface {
	IsVisible() bool
	tea.Model
}

This is useful when using models that may exist but are effectively no longer useful. This changes the existing taskprogress model to implement this interface.

Requires that #28 is merged first merged!