title: Modify text being inserted into GTK TextBuffer
body: <p>I have a <code>TextView</code> and I want to automatically convert all typed, pasted, etc. text to uppercase, is it possible? I tried adding a <code>bufferInsertText</code> handler and inserting text from there by myself, but this error message kept popping up:</p>
  
  <pre><code>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
  </code></pre>