[sailfish-browser] Improve browser history handling

Expose nextLinkId from database and use it when navigating.

Previously following kind of case caused titles to be updated
for wrong linkIds.

1) Create tab two tabs A and B (in this order)
2) Activate tab A
3) Click a link from the page of the tab A

This commit includes a small non visible improvement for
active tab title that got revealed by new WebView unit tests.
Move backForwardNavigation from model to webpage

Previously following case might have failed (difficult to reproduce)
1) Open two links in tab A
2) Open a link in tab B
3) Activate tab A and press back
4) Switch immediately back to tab B and click a link

Server latency, network speed, etc. are affecting here. Hence,
difficult to reproduce.