add basic document interaction handling (cf. issue #20)

the "import game into archive" option required by issue #20 is not yet
implemented; the handling is also not yet robust: if the user suspends
the app while the "new game" screen is still visible and then proceeds
to import another .sgf file, things will become "confused".
revise document interaction handling (closes issue #20)

issue #20 and the document interaction handling process described there
has been heavily revised since the previous commit. the new process,
implemented in this commit, is very minimal and consists only of copying
the imported .sgf file to the archive.
use new PathUtilities functions where appropriate
final touches to Inbox folder management

prevent Inbox folder from being displayed on the Archive tab; folder is
deleted on application launch; prevent user from saving a file with the
name "Inbox" (the last one is not strictly necessary since an .sgf extension
is tacked on to the file name)
use community UTI instead of app-specific UTI

sent a proposal for a community UTI specification to Anders Kierulf; using
the community UTI here instead of the previous app-specific UTI
open archived games in other applications (enhance #19)

sending archived games (.sgf files) via email is useful, but with a small
additional effort it is possible to open those game files in *any* app that
supports the SGF file type; tested with Panda-Tetsuki, but also DropBox.

as an additional bonus, the content of .sgf files can now also be copied
to the pasteboard.