bookmark-row: implement ListBoxRow.activate (enter)

implement the activate method, which gets called when enter is pressed. the click gesture handler is still there to handle middle click and modifiers. shortcuts such as ctrl+enter to open in a new tab are not implemented yet, alice said it's ok

also the function to open the bookmark of the row is moved from BookmarksPopover to BookmarkRow, because it's needed there, and what it does doesn't require getting the popover

fixes a part of #2145