Add opt-in/out support to automatic syncing

[Marlin Cremers]

Having automatic syncing enabled by default can be a breach in the user's privacy. Thus there needs to be a opt-in or opt-out system in the settings. Opt-out is preferred of course.

[Marlin Cremers]

Its a task not a enhancement this is just necessary.

[Marlin Cremers]

Added a option to enable and disable the automatic syncing. This fixes issue #68

The polling setting now also controls the automatic pushing of the clipboard content. This adds the seamless experience when using data on a different phone like a phone which requires manual pulling.

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