Don't cache tab thumbnails

When switching from one tab to another, the thumbnail of the
previous active tab might not get updated to the tab page without this fix.
[sailfish-browser] Don't load tab content from tab page as it can make engine unresponsive

Open more than five tabs (3 top most are kept alive). Close all tabs
one by one from active tab's close button from tab page. After this loading
of a new tab is failing / tabs stop loading at tab page.

Root cause is still a bit of mystery, but I think that this has
something to do with active state of web page (and/or suspend/resume).
Visibility of a web page is hidden at tab page but it should not make
a difference. What happens is that I cannot see any traces after
EmbedLiteApp::CreateView. To be more precise looks like SendCreateView
that is send through EmbedLiteAppThreadParent of EmbedLiteApp (from CreateView)
don't reach EmbedLiteAppThreadChild's RecvCreateView.
Merge pull request #182 from rainemak/loadingFix

[sailfish-browser] Don't load tab content from tab page as it can make engine unresponsive
Merge pull request #181 from rainemak/tabItemThumbFix

Don't cache tab thumbnails