Applying a time offset should be idempotent
Currently, applying a time offset in the GPX tagging module changes the timestamps of the selected images cumulatively. I propose to do this in an idempotent manner instead, i.e. the offset to EXIF data is set to the given offset. In particular, this allows to easily reset the offset to zero. Moreover, it makes the process less error-prone because of intended multiple applications of the offset.