Use box tree characters to make DisplayLists easier to scan when
printing them out.
This is what the output looked like before:
```
#### start printing display list.
"####" Stacking context at Rect(800px×608px at (0px,0px)) with overflow Rect(800px×608px at (0px,0px)):
#### SolidColor(0,0,0,0). Rect(800px×608px at (0px,0px))
#### SolidColor(0,0,0,0). Rect(784px×0px at (8px,0px))
#### SolidColor(0,0.5019608,0,1). Rect(100px×100px at (0px,0px))
#### Child layers list length: 1
"########" Stacking context at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
######## SolidColor(1,0,0,1). Rect(100px×100px at (0px,0px))
######## Child layers list length: 1
"############" Stacking context at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
```
This is what it looks like after this patch:
```
┌ DisplayList
│ ├─ Layered StackingContext at Rect(800px×608px at (0px,0px)) with overflow Rect(800px×608px at (0px,0px)):
│ │ ├─ Block Backgrounds and Borders
│ │ │ ├─ SolidColor rgba(0, 0, 0, 0) @ Rect(800px×608px at (0px,0px)) (7f926f46f1f0)
│ │ │ └─ SolidColor rgba(0, 0, 0, 0) @ Rect(784px×0px at (8px,0px)) (7f926f46f2e0)
│ │ ├─ Layered StackingContext at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
│ │ │ ├─ Backgrounds and Borders
│ │ │ │ └─ SolidColor rgba(1, 0, 0, 1) @ Rect(100px×100px at (0px,0px)) (7f926f46f310)
│ │ │ ├─ Layered StackingContext at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
│ │ ├─ Layered StackingContext at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
│ │ │ ├─ Backgrounds and Borders
│ │ │ │ └─ SolidColor rgba(0, 0.5019608, 0, 1) @ Rect(100px×100px at (0px,0px)) (7f926f46f3a0)
```
Source-Repo: https://github.com/servo/servo
Source-Revision: 4e9a888f1ac58866833418ee5e8253358918b741
