Don't use widgets for padding in Flex (#1584)

* move spacers away from being widget backed

* small cleanup

* other implementation

* fix??

* small performance improvements

* undo changes in example

* undo changes in example

* undo changes in example

* fmt

* add changelog

* mode to struct fields and add `.widget{_mut}` method on flex children

* fmt

* _even better_ nice looking stuff

* Update druid/src/widget/flex.rs

Co-authored-by: Colin Rofls <colin@cmyr.net>

* Update druid/src/widget/flex.rs

Co-authored-by: Colin Rofls <colin@cmyr.net>

* Update druid/src/widget/flex.rs

Co-authored-by: Colin Rofls <colin@cmyr.net>

* Apply suggestions from code review

Co-authored-by: Colin Rofls <colin@cmyr.net>

Author: JAicewizard

CHANGELOG.md

@@ -26,6 +26,7 @@ You can find its changes [documented below](#070---2021-01-01).
 - Warn on unhandled Commands ([#1533] by [@Maan2003])
 - `WindowDesc::new` takes the root widget directly instead of a closure ([#1559] by [@lassipulkkinen])
 - Switch to trace-based logging ([#1562] by [@PoignardAzur])
+- Spacers in `Flex` are now implemented by calculating the space in `Flex` instead of creating a widget for it ([#1584] by [@JAicewizard])
 
 ### Deprecated