Offer opening iTerm2 as a tab instead of a new window (issue #712) - #713

Reuse a terminal window that is already open, which is usually what is
wanted when a repository is sent to iTerm2, and fix two things the open
itself got wrong along the way.

• Add an "Open as tab" preference beside the terminal handler, off by
  default so the existing behaviour is unchanged
• Write the command into a new tab of iTerm2's current window when the
  preference is on, falling back to a new window when iTerm2 has none
  open
• Take the window iTerm2 opens for itself when GitX is what launched it,
  rather than asking for a second one on top
• Gray the preference out for Terminal.app, which has no Apple Event
  for opening a tab
• Call tput clear in place of clear, so opening a repository no
  longer makes iTerm2 ask about clearing scrollback history
• Raise the checkbox to sit directly under the terminal popup, so it
  reads as belonging to "Open in Terminal:" rather than to the Gist
  options below it

Screenshot

Launching iTerm2 leaves two sessions

Worth calling out because it is not obvious from the diff. Sending
createWindowWithDefaultProfileCommand: to an iTerm2 that is not running
launches it; iTerm2 opens a window of its own as it starts, and the event
then adds a second one on top. Both modes ended up with two sessions,
which is why the tab preference appeared to make no difference in that
case.

GitX now waits for the window iTerm2 opens for itself and uses that. If
none appears within five seconds it logs and asks for one, so a profile
configured to open no window on launch still works. The timeout is a
correctness knob rather than a comfort setting: firing it early would
reproduce the bug.

Measured on an Intel Mac with iTerm2 closed beforehand, driving the same
Scripting Bridge calls:

Test plan:

• PBTerminalHandlerPreferenceTests passes, with seven new cases: which
  handlers are offered the tab option, the window/tab/launched-window
  decision for every combination of running, window open and preference,
  and the shell line GitX writes into the session
• The scrollback case was written first and confirmed failing against
  clear before the fix went in
• Driven against a real iTerm2: with the box ticked, Repository > Open
  in Terminal left the window count at 1 and took the tab count from 4
  to 5; unticked, it opened a second window
• clear emits ESC[3J ESC[H ESC[2J on macOS and tput clear emits
  only ESC[H ESC[2J, measured with xxd; no announcement appeared in
  iTerm2 after the change
• ibtool reports the same notices for the changed xib as for the one
  it replaces

Closes #712