SidePanel: Migrate c/b/r/side_panel/reading_list to Lit, part 4.

In this part migrating reading_list_app.ts and icons.html. As part of
the migration also:

 - Updated CrSelectableMixin to accommodate the case where selectable
   items reside in the shadowRoot, and not in a <slot>.
 - Ported sp_shared_style.css and mwb_element_shared_style_lit.css to
   Lit.
 - Migrated sp_footer.ts and sp_empty_state.ts to Lit.

Bug: 40943652
Change-Id: I4eb527a6c28acfaadebfd4a29ed539ac2b3e82eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463215
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1290271}