[extension] Cannot type text into windowless plugins

[patwonder]

Due to the focus handler in AppIntegration.jsm, once the plugin object gains focus it is instantly blurred away. While this is OK for windowed plugins (they have their own focus), it makes some windowless plugins never gain input focus.

Unfortunately, the focus handler is required for hotkeys to work, thus cannot be easily disabled. Workaround is needed to address this issue.

Test case:
Any video on http://www.tudou.com/. Just login and try enter some comment.