Add depth parameter to pviews

This adds a depth parameter to the `pviews` command. If 0 (the default), then it simply does what it does now. If greater than 0, then it prints only up to a certain depth. This can be useful if you just want to see the top part of the hierarchy.

Example usage:

```
pviews -d 5
pviews someView -d 10
pviews 0xabcd0123 -d 1
```