[Contextcapture] Add altitude offset to imported photos

As it turns out some UAVs set absolute barometric altitude as GPS altitude for photos which causes inconvenience with adjusting using GNSS Survey points which are using correct altitudes. What we need is an option to input an known vertical offset in Contextcapture. For example if launch point is inside the area with known altitude then we could use the difference from DJI altitude values to adjust it. As you see the distance seems constant.

In MSPaint it looks like this: