add window:set: and use it to set attributes for displays.

add window:set: and use it to set attributes for displays.

We sometimes need to set DISPLAY etc so it is visible to the display
handler, particularly for ncurses.
Use new window:set: to set these on the window-core pane.
This means that editor:active-display is now always immediately after
the display pane is created, so we are closer to letting that be called
as part of creating the display pane.

Signed-off-by: NeilBrown <neil@brown.name>