Frontend: add method to WebInspector.Toolbar to obtain toolbar item for a specific panel.
Refactor the lifecycle management in ProfilesPanel. Profiles are now
deleted one-by-one, but other context (listeners, etc) is preserved.

This also fixes a subtle bug where new profile headers would not be
sent to the frontend if created after the Profiles panel is disabled
and re-enabled.

Closes #181.
Frontend: Add the capability to pin profiles to the Profiles panel so
that they persist across page loads. The pinned setting can be
overridden with an optional argument to ProfilesPanel.clearProfiles().

Closes #182.
Frontend: Implement WebInspector.ProfilesScanner, which can perform
CPU profiling scans over recordings.

Closes #161.
Bump allowed XCode version for the script that automatically opens XCode and attaches debugger.