v0.5.0

• demo: use gallerize package [#67]
• demo: auto-deploy with new tag creation with Github Actions [#69]
• dart: requires Dart >= 2.3 BREAKING [#73]
• BackdropScaffold: added frontLayerActiveFactor and backLayerScrim; replaced inactiveOverlayColor with frontLayerScrim; [#73]
• BackdropScaffold: documentation improvement for subHeader, headerHeight and stickyFrontLayer [#73]
• example: added example for variable height front layer [#73]
• BackdropScaffold: added reverseAnimationCurve [#74]
• readme: added "Accessing underlying backdrop functionalities" section [#77]