Refactored loading API
Refactored loading API

- added a new loading API in Control module to use a continuation
  based approach that is less tightly synchronized and more flexible
  than the older load_file() method (which is kept, for the time being)
- added a Downloads plugin that handles URI downloads (needs work,
  but is functional)
- FITS openers now are expanded to be able to load wildcarded indexes
  e.g. [SCI,*], [*,2], etc.
- modified the FileSelection dialogs so that they can optionally make
  a single callback with all selected files.  The old method of making a
  callback for each filepath is kept for now (but should be deprecated!)

ejeschke/ginga,naojsoft/ginga,naojsoft/ginga,pllim/ginga,ejeschke/ginga,ejeschke/ginga,pllim/ginga,naojsoft/ginga,pllim/ginga