Improve Bookmark action icons and text

This commit updates the "remove bookmark" actions to use the more
specific "bookmark-remove" icon, rather than the more general
"list-remove" icon or the older "edit-delete-bookmarks" icon which does
not exist in the Breeze icon theme. For compatibility's sake, a fallback
is added to try "edit-delete-bookmarks" if "bookmark-remove" is not
found in the active icon theme.

It also uses a longer but clearer string for the "remove all bokmarks"
context menu item.