Remove DeclarativeTab class

There are lot of small changes done. DeclarativeWebContainer
uses now only title and url from the active page and those
are also exposed to QML side. DeclarativeTabModel owns now one Tab
instance that matches to the current active tab. This is the tab that
is pushed back to model data when a new tab is activated or an active tab is changed.
Remove Tab::currentLink and Tab::setCurrentLink

This commit adds getters and setters for url, title, and
thumbnail path. Also currentLink() is there but now it is returning
the linkId similarly as previousLink() and nextLink() methods.
Rename parameter from activeTabChanged signal

Now signal has oldTabId and activeTabId parameters.
Rename DeclartiveTabModel::currentTab() to DeclarativeTabModel::activeTab()
Cleanup unit tests

Now tst_webview is the only linking against xulrunner. Other
unit tests are more less standalone but e.g. tst_declarativetabmodel
needs mocked DeclarativeWebPage.

Linking to gecko caused random bad binaries (don't completely understand)
as inside gecko RTLD_GLOBAL is used to when calling dlopen. What I saw
is that Qt SQL module found sqlite3_bind_int64 from ibmozsqlite3.so causing
unexpected behaviour when running tst_declarativetabmodel.
Merge pull request #76 from rainemak/cleanupDeclarativeTab

Remove DeclarativeTab class