Removed unneeded Mesh::add*VertexBuffer*().

It is possible to do everything with Mesh::addInterleavedVertexBuffer().
Moreover Mesh::addVertexBuffer() was dependent on vertex count, which
was counterintuitive and not always what the user wants. Also in many
times I mistakenly used Mesh::addVertexBuffer() instead of
Mesh::addInterleavedVertexBuffer() and then spent endless hours trying
to figure out what is wrong. This is now over. Thanks, brain!
Renamed Mesh::addInterleavedVertexBuffer() to addVertexBuffer().

Obvious change after previous commit, there is no other function
alternative to distinguish it from.
Improved Design Goals in documentation.
Merge branch 'master' into compatibility