GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
* Abstracts the buffer for text in ClutterText
* Allows implementation of undo/redo.
* Allows use of non-pageable memory for text
in the case of sensitive passwords.
* Implement a test with two ClutterText using the same
buffer.
https://bugzilla.gnome.org/show_bug.cgi?id=652653
