Separate Label and RawLabel

This makes label work more like other widgets; the RawLabel
widget is a thing that has a text data type (currently ArcStr)
and displays this text.

The existing Label widget is rewritten as a wrapper around
RawLabel; it is basically a sort of specialized, env-aware
LensWrap.

Author: cmyr

CHANGELOG.md

@@ -41,6 +41,7 @@ You can find its changes [documented below](#060---2020-06-01).
 - `WindowLevel` to control system window Z order, with Mac and GTK implementations  ([#1231] by [@rjwittams])
 - WIDGET_PADDING items added to theme and `Flex::with_default_spacer`/`Flex::add_default_spacer` ([#1220] by [@cmyr])
 - CONFIGURE_WINDOW command to allow reconfiguration of an existing window. ([#1235] by [@rjwittams])
+- `RawLabel` widget displays text `Data`. ([#1252] by [@cmyr])
 
 ### Changed
 
@@ -467,6 +468,7 @@ Last release without a changelog :(
 [#1241]: https://github.com/linebender/druid/pull/1241
 [#1245]: https://github.com/linebender/druid/pull/1245
 [#1251]: https://github.com/linebender/druid/pull/1251
+[#1252]: https://github.com/linebender/druid/pull/1252
 
 [Unreleased]: https://github.com/linebender/druid/compare/v0.6.0...master
 [0.6.0]: https://github.com/linebender/druid/compare/v0.5.0...v0.6.0