Split up WindowDesc to allow sub-windows and window reconfiguration.

Author: rjwittams

CHANGELOG.md

@@ -38,6 +38,7 @@ You can find its changes [documented below](#060---2020-06-01).
 - `Button::from_label` to construct a `Button` with a provided `Label`. ([#1226] by [@ForLoveOfCats])
 - Lens: Added Unit lens for type erased / display only widgets that do not need data. ([#1232] by [@rjwittams]) 
 - `WindowLevel` to control system window Z order, with Mac and GTK implementations  ([#1231] by [@rjwittams])
+- CONFIGURE_WINDOW command to allow reconfiguration of an existing window. ([#1235] by [@rjwittams])
 
 ### Changed
 
@@ -56,6 +57,7 @@ You can find its changes [documented below](#060---2020-06-01).
 - `Movement::RightOfLine` to `Movement::NextLineBreak`, and `Movement::LeftOfLine` to `Movement::PrecedingLineBreak`. ([#1092] by [@sysint64])
 - `AnimFrame` was moved from `lifecycle` to `event` ([#1155] by [@jneem])
 - Contexts' `text()` methods return `&mut PietText` instead of cloning ([#1205] by [@cmyr])
+- Window construction: WindowDesc decomposed to PendingWindow and WindowConfig to allow for sub-windows and reconfiguration. ([#1235] by [@rjwittams])
 
 ### Deprecated