Rename BaseState to WidgetState

This is motivated by a desire to avoid some confusion with a
future patch that adds a RootState struct to unify some of the
other stuff shared between various contexts.

Author: cmyr

CHANGELOG.md

@@ -138,6 +138,7 @@ This means that druid no longer requires cairo on macOS and uses Core Graphics i
 - X11: Refactored `Window` to support some reentrancy and invalidation. ([#894] by [@xStrom])
 - Refactored DPI scaling. ([#904] by [@xStrom])
 - Added docs generation testing for all features. ([#942] by [@xStrom])
+- Renamed `BaseState` to `WidgetState` ([#969] by [@cmyr])
 
 ### Outside News
 
@@ -217,6 +218,7 @@ This means that druid no longer requires cairo on macOS and uses Core Graphics i
 [#961]: https://github.com/xi-editor/druid/pull/961
 [#963]: https://github.com/xi-editor/druid/pull/963
 [#967]: https://github.com/xi-editor/druid/pull/967
+[#969]: https://github.com/xi-editor/druid/pull/969
 
 ## [0.5.0] - 2020-04-01