Switch to only supporting scans from mail files

Mail files can be parsed more reliably than plaintext, and
the way plaintext was being handled before was terrible
anyway.  It may be reimplemented in a better way someday,
but don't hold your breath.
Ensure the mail file is actually a system survey
Output file being processed to commandline
More robust handling of radius (commas, Lm)
Dateutil is no longer used
Correct --tl option
Fix bug with gas giant support
API to search by exact resource name
Support custom planet names
Correct file header parsing
Add support for scanning mail cache
Update the README to account for the mail support
Fix bug with processing new scans of old locations
Use combobox for resource name
Allow searching by orbit zone
Permit multiple selection for orbit zone
Remove redundant line
Re-order orbit zone list
Allow searching by body kind
Update status bar when search initiated