Integrate git-ls-files in the file view, deleted files won't show anyway, but the...

Integrate git-ls-files in the file view, deleted files won't show anyway, but the rest works ok.

- src/giggle-git-list-files.[ch]: support all file status types. (giggle_git_list_files_get_file_status): new public function to check the status of a given file (relative path from the project root). Add GiggleGitFileListStatus enum
- src/Makefile.am: add giggle-git-list-files.h to glib-mkenums .h files
- src/giggle-file-list.c: store the file status in the model, remove COL_MANAGED in favour of COL_FILE_STATUS and make pixbuf association based on the file status too.