Add some cppunit-based tests for BTextView.

Note that this does not reproduce #8447. I could not get more precise
details from involved people about how to reproduce this, and the patch
was already applied anyway, so I'm stopping my investigation for now.
Feel free to complete this with more test cases.
Add the ALM headers to the haiku_devel package.

We provide the library, so why not also provide the development headers
for it?