Put an "add bookmarks" button on sidebar page and redo button UI

This MR has three inter-related commits:

Author: Nate Graham <nate@kde.org>
Date:   Fri Aug 27 09:42:02 2021 -0600

    Put an "Add Bookmark" button on the Bookmarks sidebar
    
    Previously, the only ways to add a bookmark were with the menu item or
    keyboard shortcut. This commit adds a button at the bottom of the
    bookmarks sidebar to do the same thing, which seems like a logical place
    for such functionality.

    setupViewerActions() was moved to earlier in the setup process to ensure
    that m_addBookmark is assigned by the time the bookmarks sidebar is
    created, since it gets passed that action in its constructor.

    BUG: 357625
    FIXED-IN: 21.12

Author: Nate Graham <nate@kde.org>
Date:   Thu Sep 23 14:16:49 2021 -0600

    Improve UI for Bookmarks sidebar page buttons
    
    Currently there are two tiny icons-only buttons with very similar icons
    (which may even be identical in some icon themes). One of them is
    checkable, and the other one triggers an immediate action. And there is
    lots of unused space to the right of them.
    
    This commit overhauls them in the following way:
    - The toggle button becomes a checkbox with visible text, and is
      inverted; now when checked, it shows all bookmarks
    - The checkbox moves above the view and search field, to communicate
      that it affects everything below it
    - The button to add a bookmark gets some visible text
    
    This should make the UI much clearer, at the cost of one extra line of
    vertical space from the bookmarks list.

Author: Nate Graham <nate@kde.org>
Date:   Mon Sep 27 12:17:15 2021 -0600

   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.

Sidebar, before-and-after:

Before	After

Context menu, after:

This supports !463 (merged) by letting us not add an "Add Bookmark" item to the hamburger menu because it will already be somewhere visible in the main UI.

cc @teams/vdg @felixernst