Update security slider to follow NoScript protocol change

ma1 [ticket:26128#comment:15 wrote]:

Please notice that NoScript 10.1.8.17 does change its message handling to slightly abstract it, simplify it and fix some subtle bugs.

As a consequence, the property by which all the messages (including updateSettings)identify themselves is not called "type" anymore, but "_messageName".

In order to work with 10.1.8.1.17 and above, the Security Slider must therefore send this the updated policy in an object with a "_messageName": "updateSettings" property (alone, or in addition to the existent "type": "updateSettings" one for backward compatibility).

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