delete or remove download item in download manager would cause random crash
delete or remove download item in download manager would cause random crash

Root cause is wrong usage of download manager's API
in exporting extension API for OMA download. Delete or
remove download item would send "modelchanged" to observer,
which would trigger this bug.


Former-commit-id: 50e7ab548250330c646282b3969ada502bee85c1
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser