[WIP] Message splitting

[KoKuToru]

for #222

[KoKuToru]

sometimes generates a

(gtox:26182): Gtk-WARNING **: Invalid text buffer iterator: either the iterator is uninitialized, or the characters/pixbufs/widgets in the buffer have been modified since the iterator was created.
You must use marks, character numbers, or line numbers to preserve a position across buffer modifications.
You can apply tags and insert marks without invalidating your iterators,
but any mutation that affects 'indexable' buffer contents (contents that can be referred to by character offset)
will invalidate all outstanding iterators

warning.. right now size of input is limited by a check in m_buffer->signal_changed().
It looks like gtk doesn't except the buffer to change on this signal.. might need to reject key inputs/pasted text in a different way..