Relay domcontentloaded once the root window receives it

Currently sailfish-browser observes "embedlite-before-first-paint" for taking capture of the web content for a tab thumbnail. When real tabs are used (multiple mozviews), context wide observing is not nice.

An alternative to "domcontentloaded" could be "MozAfterPaint" but as that event is triggered quite frequently; pan, flick, content animates itself (e.g. advertisement) I think that "domcontentloaded" gives good enough result for the purpose. We can revise this if needed. What do you think?