Add parameter to open nodes of DocumentTree to given level/depth

Simon Urli 

In my mind, the parameter would work as follows:

openToLevel="1" would display:

- Help
- Bar
- Foo
- Sandbox

openToLevel="2" would display:

- Help
   - Help Page 1
- Bar
   - Sub Bar
- Foo
   - My Foo
- Sandbox
   - Sandbox Test Page 1
   - Sandbox Test Page 2
   - Sandbox Sub Space

openToLevel="3" would display:

- Help
   - Help Page 1
      - Chapter 1
      - Chapter 2
      - Chapter 3
      - Chapter 4
 - Bar
   - Sub Bar
      - Sub Sub Bar 1
      - Sub Sub Bar 2
- Sandbox
   - Sandbox Test Page 1
      - Sub Test Page
   - Sandbox Test Page 2
   - Sandbox Sub Space
      - Sub sandbox 

This is under the assumption that there is no "openTo" parameter provided. If botch parameters are provided, then the effect of the "openToLevel" could be limited to the branch selected by the "openTo" parameter.

I didn't put too much thought into the name of that new parameter, and I'm generally bad at naming things. If you feel the name is confusing, feel free to change it