This commit adds some plumbing for describing the cursor shape
(block, line, blinking etc) and visibility, and feeds that through
the mux and render layers.
The renderer now knows to omit the cursor when it is not visible.
previously we would only ever copy to the clipboard when the selection
was changed.
Now we respect the Copy keypress and have it re-copy the selection
into the clipboard.
This is a bit of a large commit because it needed some plumbing:
* Change mux creation to allow deferring associating any domains,
and to change the default domain later in the lifetime of the
program
* De-bounce the empty mux detection to allow for transient windows
during early startup
* Implement a bridge between the termwiz client Surface and the
frontend gui renderer so that we can render from termwiz to
the gui.
* Adjust the line editor logic so that the highlight_line method
can change the length of the output. This enables replacing
the input text with placeholders so that we can obscure password
input
