Cleanup SC.SegmentedView overflow menu.

There were two problems here. When the overflow menu was removed, it was
never being destroyed, causing a memory leak. Second, when the segmented
view was destroyed, the overflow menu was not being removed, so you would
end up with a dangling menu in this case, causing crashes if you tried to
interact with it.
Prevent SC.MenuPane from handling escape/return when not visible.

SC.MenuPane would handle escape and return key presses even if not
visible, as long as the event originated from a visible view such as
SC.PopupButtonView. This fix will only handle escape and return if the
menu itself is visible.
Fixed issue with property chain notifications.

Property chains would by notified during teardownEnumerablePropertyChains,
but this could result in computed properties trying to access the array
before it was finished mutating. This was observed with SC.ChildArray's.

Instead, wait until setupEnumerablePropertyChains to notify, and always
notify with the assumption that changes were made.
Fix bad merge in previous fix.

This is purely a merge conflict, no need to push upstream.
Fix some additional memory leaks.
Fix crash when double clicking on menu items.

If a menu item was set to flash before performing the action, a second
click could get handled after the root menu was removed, causing a crash.
Fix unknown exception in IE.

Sometime IE would throw an unknown exception when trying to call
getBoundingClientRect. Catch it so we don't crash.
Fix user agent detection for Opera.

Since Opera switched to using Chromium as its base, it has Chrome in the
user agent string and was being detected as Chrome. They also changed
from using Opera to OPR as the browser name in the user agent.
Fix crash when removing highlight from menu items with submenus.

checkMouseLocation is invokeLater'd. That means the menu could have been
closed by the time it runs, but it was not null checking, causing a crash.
Adds an unbind function.

unbind is the opposite of bind, allowing the removal of bindings created
manually with bind.
Bumping to SC 1.9.0.19.