Set database image metadata based on exif metadata contained in image

[dersmon]

We already include https://github.com/mattiasw/ExifReader in FieldDesktop.

Values that could potentially be filled automatically:

• imageRights
• draughtsmen (Array), values have to map to existing project staff members as defined in the project's properties

[dersmon]

See See: #163

Things still todo:

• Additional information in the interface: What fields in Exif etc. will be parsed, currently it is not explained in detail.
• Check the project configuration if draughtsmen and creationDate are even configured for the image categories.
• Instead of using a hardcoded label for the draughtsmen field, read the value from the project configuration, basically replace this: https://github.com/dainst/idai-field/pull/163/files#diff-ebc465674af1d09ed8527a2ff40ad384085d5203facfe9052b9dda963b6a5834R53-R55 with the label defined in the project's configuration.