Bookmarks: Added BookmarksTree class that manages all bookmark's view types.
-BookmarksSidebar, BookmarksWidget,BookmarksManager and ... use BookmarksTree
-Added 'New Folder...'
-Fixed some issues
Bookmarks: Don't allow creating folder with translated name of "bookmarksToolbar", "bookmarksMenu" or "unsorted"!
Added 'Expand All' and 'Collapse All' buttons to bookmarks manager.
Move connectons from .ui to .cpp
Bookmarks: when deleting lastFolder set it to default.
Merge branch 'bookmarks_work' of git://github.com/srazi/qupzilla into srazi-master
Updated German translation

Reviewed by J. Hooverman
Coding style
[Qt5] Ported all plugins to Qt5 plugin system
[Fix] Saving '+' character in passwords/usernames

Closes #703