Capability to set UI.Input as focused by default

[maluoi]

Describe the feature

On some occasions, it can be nice to have a UI.Input element focus by default. Not sure if this applies well to elements other than UI.Input yet, so it could be either specific to UI.Input, or a more general focus setting API. SK currently has no keyboard traversal, so the concept of keyboard focus is unique to UI.Input so far. If keyboard UI traversal happens in the future, the generic focus API could be preferred.

An example use-case

A rename button that spawns a window with just a UI.Input.
A login window that pops up with username/password fields.

In both these cases, what the user should do next is clear, and requiring the user to select the Input is somewhat redundant. Popping the keyboard right away could streamline the process.