added ObjectKeys type helper function

src/mol-util/type-helpers.ts

@@ -20,3 +20,7 @@ export type ArrayCtor<T> = { new(size: number): { [i: number]: T, length: number
 export type AssignableArrayLike<T> =  { [i: number]: T, length: number }
 
 export type NonNullableArray<T extends any[] | ReadonlyArray<any>> = T extends any[] ? NonNullable<T[0]>[] : ReadonlyArray<NonNullable<T[0]>>
+
+export function ObjectKeys<T extends object>(o: T) {
+    return Object.keys(o) as (keyof T)[]
+}