Web activity: quit the GTK main loop when activity is destroyed
Web activity: quit the GTK main loop when activity is destroyed

Fixes #4603 .

In GTK activities, this is done by _ActivitySession when the activity
unregisters itself, in _complete_close method.

Making the web activity aware of session logout will be handled in
another patch for the shell, see http://bugs.sugarlabs.org/ticket/4611

tchx84/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,samdroid-apps/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,manuq/sugar-toolkit-gtk3,puneetgkaur/backup_sugar_sugartoolkit,godiard/sugar-toolkit-gtk3,samdroid-apps/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,samdroid-apps/sugar-toolkit-gtk3,tchx84/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,samdroid-apps/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,manuq/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,puneetgkaur/backup_sugar_sugartoolkit,tchx84/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,manuq/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,puneetgkaur/backup_sugar_sugartoolkit,puneetgkaur/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3