Inhibit suspend while doing text to speech

When suspend/resume is enabled, and sugar is doing tts
in a long text, suspend can make the computer sleep
in the middle of the text.
Having support to inhibit suspend/resume in the toolkit
allow us to close this old OLPC ticket [1]

[1] http://dev.laptop.org/ticket/11830

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>