Dev:Packed files

As requested here, Synfig Studio needs functionality that would collect all files referenced in a sif and place them all in a tgz for sending elsewhere or publishing animations in source form.

My proposal is that we need to define additional format for such files, containing all referenced data inside. Synfig should be able to open and save those files. This format could be a simple ZIP archive with some extension (.sifp, for example).

The steps to implement:

1. Define structure of packed file format.
2. Implement file saving
3. Implement file loading
4. Add Media Panel which allows to manage packed data. Basic functions:
   1. Extract packed file
   2. Remove packed file
   3. Convert packed file to external unpacked (external link)
5. Add "Link" option to Import dialog. This option defines if imported file will be linked as external or packed into current file.
6. Modify Media Panel - list the external files there too. Add functions for external files:
   1. Convert external file to packed