Session export; History export

[HT-7]

There needs to be a native session export feature in Kiwi browser so users can start with a fresh session without losing the prior state.

The session is saved as a file into the Download folder where it can be viewed from a computer or backed up to a USB stick, so work can be done between devices.

The Chrome extensions for this purpose do not work with Kiwi. It would probably take less than ten minutes to implement this vital feature.

Since the beginning of web browsers, not having this feature natively was a big mistske. Let's do it better. Let's set a positive example in the world of web browsers.

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This also applies to history exporting, because Chromium only keeps history for three months since version 37 (August 2014). An extension could be used for that, but due to all extensions suddenly disappearing (#860), that is not reliable anymore. Therefore, this important feature needs to be a native feature. It's time to welcome power users to Kiwi browser.