wpf: make the cursor blink

[ghost]

Summary of the Pull Request

Adds a blinking cursor to the WPF terminal control

Validation Steps Performed

VS terminal was patched with new bits to verify cursor only blinked when it had keyboard focus.

[DHowett-MSFT]

I don't expect there's an answer to this that helps y'all ship sooner, but is there an architectural improvement we could take on that would make it so you don't need a p/invoke down into C++ just to tell the cursor to blink? 😄

(request changes is for the method names alone.)

[ghost]

I don't expect there's an answer to this that helps y'all ship sooner, but is there an architectural improvement we could take on that would make it so you don't need a p/invoke down into C++ just to tell the cursor to blink? 😄

(request changes is for the method names alone.)

Registration-free COM might work, but I'm not an expert on how to set that up. We at least need some of the blink handling happening in the managed code since that has access to the dispatcher timer.