Add "tool browser" to manage a pane's hierarchy and scripts

[marceltaeumel]

This would likely be a good additon for (live) demonstrations and screencasts.

At the moment, users can explore complex tools (i.e. multiple panes, views, and pane views) as follows:

• browse the pane hierarchy via the outermost pane halo and the #paneHierarchy script
• pick any pane of that complex tool and view script w/ current input objects

However, both ways for themselves are rather tedious:

• pane hierarchy lacks script editing features
• halo invocation can only show selected parts of the tool

Thus, we need a kind of tool browser that combines overview, navigation, and modification in a rather stable form to help users:

• explore complex tools
• verify functionality and robustness of complex tools
• follow demonstrations and screencasts
• ... build even more complex tools ... 😄

[marceltaeumel]

See #239