Bookmarks: improvment and fixed some issues.

First commit 8f2b9a0 : 
1- Lots of bookmarks view stuff moved to new class BookmarksTree. And BookmarksSidebar, BookmarksManager and Bookmark's ComboBoxes use BookmarksTree for their view.

2- Added "New Folder" functionality to Bookmark's ComboBoxes. (i.e. `BookmarksWidget` and `bookmark this page dialog` and `bookmark all tabs dialog`)

3- Fixed some bugs, but I don't mention them, if you want I can add some comments.

Third commit 01cc581 : 
I think, when there are lots of folders expandAll and collapseAll are useful, what do you think?