Add tabsmodel as central place to control the gui

Advantages:

• Model information is not duplicated between the qml part (QtQml.Models) and the browserManager::m_tabs
• Changes do not need to be synced with browser manager anymore
• properly testable (and test implemented)
• A lot less logic in the ui (qml part)
• No json inside the ini file

Drawbacks:

• isMobile is not saved to disk anymore to be able to use a simple ini list (doesn't matter much to me, please comment if it does to you, ideally with idea how to store it)

General Changed:

• BrowserManager is now a singleton with a static instance getter, to share one QSettings instance throughout Angelfish

This probably needs some manual testing since it touches almost every core functionality. Commits will be squashed once this is ready to be merged.