[sailfishbrowser] Store home page in a configuration file.

So that it can be read by other apps, e.g. simkit needs this.
I believe that empty home page will be handled in simkit side. Also it must be handled in browser settings once we get this setting exposed to there, but for that we'll need e.g. tab setting (always restore, don't restore after quitting browser).

We could still guard this like...
```
QString homePage = m_homePage.value("http://jolla.com").value<QString>();
return homePage.isEmpty() ? "http://jolla.com" : homePage;
```