frontend: update displayed pin when manually typing lat/lon

[ES-Alexander]

The Latitude and Longitude text boxes allow manually specifying an origin / position, but doing so does not update the pin position in the UI, which is unintuitive and may cause some usage issues.

Raised in this forum post.

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As something of a side note, it could be useful to have an indication of whether the origin or position is currently being set (which would make the actual behaviour clearer). Would presumably require checking if origin is set before creating the interface, then setting the map title and the submit button text to use 'EKF origin' or 'position' as relevant.