Use classes instead of IDs - Part two

The IDs are kept in some cases to keep compability with child themes.

* #slider -> .cakifo-slider
* #topbar -> .topbar
* #headlines -> .headline-columns
* #recent-posts -> .recent-post-columns
* #sidebar-primary -> .sidebar-primary
* #sidebar-secondary-> .sidebar-secondary
* #sidebar-subsidiary -> .sidebar-subsidiary
* #sidebar-after-single -> .sidebar-after-single
* #sidebar-after-singular -> .sidebar-after-singular

See 4c70e2a267e6a21322141c1107c8d4b7ba627955 and
0abf8d1b8ba90f0372bb9140dd5152678f1ab9ab