Add Nuvola._log func to JS API

It will be used as a fallback for console.log in App Runner process.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Provide useful functionality in bare global object

console.log()
console.debug()
alert()

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>