battery properties (#903)

* Battery params

* move category

* cleanup

* use property

* fix level

* fix battery computation

* fix level comp

* docs

libs/ev3/ns.ts

@@ -1,6 +1,6 @@
 
 //% color="#68C3E2" weight=100 icon="\uf106"
-//% groups='["Buttons", "Screen"]'
+//% groups='["Buttons", "Screen", "Battery"]'
 //% labelLineWidth=60
 namespace brick {
 }