Update the back button to better work with tabs.
Currently, if a tab has no page to go back to, it loads a blank page. This loading is actually a forward motion, so the current page becomes the new back. Hitting back again causes the previous page to load. A user can cycle between them ad infinitum by continuing to hit back.
This was a cludgy result of trying to fix behavior when implementing tabs. Previously, if the WebView in a tab could not go back, it would kick the command up to the system, which would reopen the calling intent (either another app or the desktop). This felt very odd if hitting back on a tab that one didn't realize was already at the beginning because one would be thrown out of Privacy Browser even if other tabs were open.
I think that the preferred behavior should be that, if the the current tab has reached the first entry and there are more than one tabs, Privacy Browser should close the current tab when back is pressed. If there is only one tab, and back is pressed when reaching the beginning of the list, then the command should be passed onto the system.