Allow to change the bookmaked status of the current page

I have reimplemented https://invent.kde.org/plasma-mobile/plasma-angelfish/-/merge_requests/79 and prefer this much more.

It is offloading url status to separate class which can be expanded in future as needed. On QML side, all is simple and we don't have to track excessive changes in properties.

It is also possible to have separate bookmarked indicator on navbar, if wished for. I tried and didn't like it though. But, as all logic is in C++, it is possible if better visual solution will be found.

Please review. Much of the code is shared with the earlier MR.

If you agree, I will close !79 (closed) and instead would use this.