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