mac: added 'mount filesystem' action to the mac gui

mac: added 'mount filesystem' action to the mac gui

this adds an action to the dock menu and to the file menu (when visible)
"Mount Filesystem".  This action opens a windows offering the user an
opportunity to select from any of the named *.cap files in their
.tahoe/private directory, and choose a corresponding mount point to mount
that at.

it launches the .app binary as a subprocess with the corresponding command
line arguments to launch the 'tahoe fuse' functionality to mount that file
system.  if a NAME.icns file is present in .tahoe/private alonside the
chosen NAME.cap, then that icon will be used when the filesystem is mounted.

this is highly unlikely to work when running from source, since it uses
introspection on sys.executable to find the relavent binary to launch in
order to get the right built .app's 'tahoe fuse' functionality.

it is also relatively likely that the code currently checked in, hence
linked into the build, will have as yet unresolved library dependencies.
it's quite unlikely to work on 10.5 with macfuse 1.3.1 at the moment.