BMenuItem: Remove ourselves from the super menu on destruct.

This fixes the (intermittently) crashing test added in the previous commit,
and should also fix #12024 and #14348.

Note that this is a slight behavioral departure from BeOS, though since
BeOS crashed when this was done previously, it shouldn't cause any
other problems.

Change-Id: I90b6132ff7741b8d6cb601375a9b11fc3ffacb40
Reviewed-on: https://review.haiku-os.org/541
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

IK documentation update

* Add \since directive to each method.
* Add documentation for BScrollBar and BScrollView classes.
* Title Case group titles.
* Some other minor documentation updates.

Style fixes to BMenuItem::GetContentSize and update docs

Rename width and height to _width and _height matching the header.

Update the documentation for this method as well.

Document the BMenuItem class in the Haiku Book.

IK documentation update

* Add \since directive to each method.
* Add documentation for BScrollBar and BScrollView classes.
* Title Case group titles.
* Some other minor documentation updates.

Style fixes to BMenuItem::GetContentSize and update docs

Rename width and height to _width and _height matching the header.

Update the documentation for this method as well.

Document the BMenuItem class in the Haiku Book.