Can edit input fields for interactions in exploration editor

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Create or edit an exploration containing a text input, number input, or code.
2. Press tab until a text cursor appears in the code box or on the text/number 
input lines.
3. Type.

What is the expected output? What do you see instead?
It should not be able to focus these interactions. Typing provides input that 
is validated, though read-only interactions should not be able to receive input 
(even if it is not used).

Original issue reported on code.google.com by bhenn...@google.com on 29 Jul 2015 at 1:58