Debug tools: new "Stack Tool" window, ShowStackToolWindow()

[ocornut]

Introducing the stack tool

(We've been using this internally as part of the unreleased automation feature but recently managed to redesign in a way allowing us to move it to the main library while not incurring significant cost.)

The tool run queries on hovered id to display details about individual components that were hashed to create an ID. It helps both understanding the ID stack system and debugging potential ID collisions.