gui/macapp: rough cut of ui tweaks; configurability, auto-mount

gui/macapp: rough cut of ui tweaks; configurability, auto-mount

chatting with peter, two things the mac gui needed were the ability to mount
the 'allmydata drive' automatically upon launching the app, and open the
Finder to reveal it.  (also a request to hide the debug 'open webroot' stuff)

this (somewhat rough) patch implements all the above as default behaviour

it also contains a quick configuration mechanism for the gui - rather than a
preferences gui, running with a more 'tahoe' styled mechanism, the contents
of a few optional files can modify the default behaviour, specifically file
in ~/.tahoe/gui.conf control behaviour as follows:

auto-mount (bool): if set (the default) then the mac app will, upon launch
automatically mount the 'tahoe:' alias with the display name 'Allmydata'
using a mountpoint of ~/.tahoe/mnt/__auto__

auto-open (bool): if set (the default) then upon mounting a file system
(including the auto-mount if set) finder will be opened to the mountpoint
of the filesystem, which essentially reveals the newly mounted drive in a
Finder window

show-webopen (bool): if set (false by default) then the 'open webroot'
action will be made available in both the dock and file menus of the app

daemon-timout (int): sets the daemon-timeout option passed into tahoe fuse
when a filesystem is mounted. this defaults to 5 min

files of type (int) much, naturally contain a parsable int representation.
files of type (bool) are considered true if their (case-insensitive) contents
are any of ['y', 'yes', 'true', 'on', '1'] and considered false otherwise.