In most cases, we just want Component<any, Env>. But since it was so
annoying to have always the type Env, we actually used
Component<any,any> everywhere.
With this commit, the generic types have a default (and their order is
swapped), so we can simply use Component in most cases, and
Component<Props> when we want to type the props.
It now takes two arguments: parent (optional, only for non-root
components) and props (optional). In the case of the root component,
the env is taken from the config (config.defaultEnv). If it doesn't
exists, the default env is created on the fly.
Closes#306
Géry Debongnie