Modify the WindowSize method of the Tty interface to return a WindowSize
object. This object includes the window size in pixels, for use in
direct drawing of sixels. Terminals that do not support this feature
will have pixel sizes of 0,0.
fixes#449 Lost keyboard input after suspend on Windows 10 PowerShell
fixes#148 Make tcell usable with any io.Reader and io.Writer
This introduces a new Tty interface so that applications can supply
their own implementation. This should facilitate work for applications
that wish to provide e.g. a webasm version of the terminal, or that need
to use different kinds of file plumbing.
