Add Terminal::DrawState wrapping and add method to read width/height of Terminal::Framebuffer

Terminal::Framebuffer has a public instance field (ds) of type Terminal::DrawState. Terminal::DrawState has two instance methods get_width() and get_height(), which contain the current dimensions of the associated Terminal::Framebuffer.

While the existing SWIG spec exposes the resize(width, height) method on the framebuffer, there is no way to read those dimensions for a framebuffer instance. Need the ability to read those dimensions (and thus detect changes) to be able to handle window resizing events.