popup-commands: don't cancel a download that hasn't even been started

Cancelling the "save link as" dialog is calling ephy_download_cancel()
even when ephy_download_start() has not been called. This causes a
critical warning in webkit.

https://bugzilla.gnome.org/show_bug.cgi?id=681010
ephy-history-service: prevent double call to execute_quit()

execute_quit() is called already when the QUIT message is received,
there is no need to call it after the thread loop quits.

https://bugzilla.gnome.org/show_bug.cgi?id=680529
[release] 3.4.3

Require WebKitGTK+ 1.8.2
Assamese translation updated
Update French translation