Unify the summary tree and widget details tree

[elliette]

Currently, the inspector contains two trees:

Inspector summary tree

• Labeled as the “Widget Tree” in the UI
• This tree shows only widgets that were created in the user’s project

Inspector details tree

• Labeled as “Widget Details Tree” in the UI
• This tree shows all the children widgets of the currently selected widget

The two trees can be unified, so that all the widgets are viewable in a single tree. As part of this, we can add styling distinguish between different types of widgets:

Widgets that were created in the user’s project

• These are the widgets that are currently in the summary tree

Widgets that were created in the framework or other external packages

• These widgets can be collapsed using an affordance like <8 more…>