Focus tree inspector

[Albert221]

Similar to #1397 and #481, it would be helpful to have the inspector of focus nodes tree in a GUI like devtools, in addition to debugDumpFocusTree

[kenzieschmoll]

Additional request for a focus management tool from a g3 customer.

[loic-sharma]

@jacob314 and I created a hacky prototype:

This works by swapping out the widget inspector's diagnostics node to use the WidgetsBinding.focusManager instead.

TODO:

1. We'll need to update device inspector selection to work for focus nodes
2. The tree and console should use the focus node's debug name instead of child N
3. More...