Bunch of tiny improvements - #3433

- Tweaked styling to add a little extra shadow and be more rounded to
  match other UI areas.
- Added slight horizontal inset when in right sidebar to prevent shadow
  being cut-off in most cases.
- Added logic to "drop upwards" if dropping down would take the menu
  offscreen.

- Removed background track line since it would darken entity item bars.
- Updated item spacing to be a bit tighter.
- Updated action hover styles to be a bit lighter, and visible on dark
  mode, to fit rest of system.

- Removed border and rounded list item styles to make hover states have
  less edge detail and to align with other UI elements.
- In expanded-detail view, removed space used for entity description if
  there is not description content existing.

- Removed harsh theme color border between search and content.
- Prevented intermediate focus on list container to align arrow & tab
  behaviour, and to get to content quicker.

Loading animation would show in an unready state due to animation-delay
on components. Updated to a negative delay to ensure elements were in
correct positions right away upon show.

Animates the height on breadcrumb dropdown menus to transition to the
loaded animations quicker. Includes a new animation helper for doing
similar tasks in future.

Would cause effect where scroll area would be cut of by spacing which
looked a bit strange. This retains the same padding sizes but cuts the
content at the header or top of viewport.

Would cause a slight dark area in top left of view while hidden.

Specifically updates dropdown search and user-search implementation,
although does affect all inputs.
Decouples breadcrum and select-style dropdown search toggles.

Addresses #2678

- Updated many items to be flexbox-based.
- Updated & aligned hover states across header bar items.

Previously the desktop-visible items would style different when collapsed
into the expanded dropdown menu, compared to existing items.

- Improves animation smoothness
- Changed animation slideup/down animations to use max-height instead of height
  to better avoid jutter at the end.
- Cleaned spacing to match page items in books listing.

Was extra space showing due to structure changes and flex gap.

- Now has a hover state to match other items.
- Now spans the full sidebar with like other items.
- Also updated chapter-toggle to a chapter-contents component, following
  the newer component system.

- Allows easier accessibility of inline attachments.
- Introduces a new split-icon-list-item thingy to support such cases
  where only part of the button is actually linked.