Tagging functionality

[fzampella-huawei]

Introduce a “Add tag” button that allow users to highlight the current position observation. The position will be introduced into the gnss_data array of the Trajectory (protobuf definition) with the following data:

• relative_timestamp, latitude, longitude, altitude: current values
• provider: “fusion”

I recommend we use an interface like these ones:

• steplogger
• steplogger_fullscreen