Display bookmarks in the workarea.

Display bookmarks in the workarea.

The bookmarks are added as virtual elements in display Rows. Bookmarks
are shown with circled numbers. A new color "bookmarks" has been
added. Currently bookmark 0 (the return position) is not displayed
because it is very disturbing in practice.

To make this work, a new method BookmarksSection::bookmarksInPar
retuns the list of bookmarks in a paragraph along with their position.

Force redraw when using bookmark-save and bookmark-clear.

Fixes bug #2496.