Add drag-and-drop new entry stubs for OSX.

Although this pull request still needs to be refactored against `ebib-add-entry` with some error checks/alternatives for other os/window systems, it should give you an idea what I'm thinking.

It allows you to drag-and-drop a file, or directory of files (which may contain directories of files, etc.) and add them as temporary entries. The file paths are added to the `ebib-standard-file-field` via `ebib-file-relative-name`, so it uses the relative name first, then the full file path if not found in the search directories. Temp entry keys are: `<new-entry>, <new-entry-1>, <new-entry-2>`, etc.

This is more of a workflow enhancement than anything else. Typically I browse, read, download, and organize papers before filling in the actual bibtex info (but this is of course a personal preference). It's been nice to use ebib in the organizational stages.

Perhaps down the road, in my own `.emacs`, I can add convenience functions that automatically copy files from my downloads folder into my `ebib-file-search-dirs` when I drag them over to ebib.

The drag-and-drop binds to [ns-drag-file](https://www.gnu.org/software/emacs/manual/html_node/emacs/Mac-_002f-GNUstep-Events.html), I've only tested on OSX, so I'm unsure how it affects Linux or Windows.