Add option of saving subsurface userid in xml
Fixes #473

Changes Made:
1. Choose if User ID is to be saved during import of divelogs from
   subsurface webservice. Save function must be called once for change
   to take effect.
2. Checkbox retains the last set option or as per present xml.
3. Suitable modifications made in parse-xml.c
4. If userid is present in xml, saving option is enabled, else
   disabled. For manually enabling it, use the webservices dialogbox.
5. The userid in the xml file does not override the settings until
   manually done so via webservices dialogbox.
6. Userid, if present in the xml, is shown in the download dialogbox.
   But it doesn't override the settings of subsurface. If userid is
   absent, then the saved userid is shown in dialogbox.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>