Visually differentiate bookmark icons

This MR has two commits:

Author: Nate Graham <nate@kde.org>
Date:   Fri Aug 20 13:12:45 2021 -0600

    Add new bookmark-new and bookmark-edit icons
    
    Before, these icons (and the icons symlinked to them) were simply using
    the base icon, which was inappropriate at best and misleading at worst
    in the case where multiple actions that had different things were all
    adjacent to one another; in this case you would see several of the same
    icon but they all did different things! Now all the different bookmark
    icons are visually differentiated from one another.

Author: Nate Graham <nate@kde.org>
Date:   Fri Aug 20 12:08:46 2021 -0600

    Re-organize bookmark icon symlinks
    
    This commit makes `bookmark.svg` the base icon, with everything else
    pointing to it. the "add" style icons point at `bookmark-new` to
    facilitate easily replacing just that icon when an appropriate "add" style
    icon is created.

The end result, light color scheme:

Dark color scheme: