[sailfish-browser] Fix empty tab thumbnails not to appear. Fixes JB#1679...

...1

This commit fixes two cases:
1) Multiple tabs open, active tab is closed from tabs pages, and loading
   finishes so that tabs page is still active, tabs page popped from
   the PageStack, browser pushed to home screen, and open an external link.
2) Open browser, push it to home screen, and open an external link,
   check thumbnails from tabs page.

This commit adds also dumpTabs helper that can be used when debugging tabs.