fix play deploy

1.5.6 Changes (halfhp#81)

* adds convenience methods for saving and restoring pan / zoom state
* halfhp#80 Target SDK 28
* readme updates

remove unused button from simple xy plot

1.5.4 Changes (halfhp#74)

* halfhp#69 Fixes resize etc. for Y_VALS_ONLY format for SimpleXYSeries.

* halfhp#73 Fixes issue with XYGraphWidget not laying out on size change.  Also adds Widget.onResize to efficiently detect when new layout dimensions may need recalculation.

* Adds license and issue template for Github display