Improve handling of links

- Remove special handling of footnotes
- Open all links by default
- Reader can return `true` to prevent default action
- Expose link handling as a method, which the reader can use to set up
  links outside renderers (e.g. in popup footnotes)

ui/popover.js

@@ -1,63 +0,0 @@
-const clamp = (min, max, x) => Math.min(max, Math.max(min, x))
-
-const arrowHeight = 10
-const arrowWidth = 20
-const radius = 6
-
-const createSVGElement = tag =>
-    document.createElementNS('http://www.w3.org/2000/svg', tag)
-
-const createArrow = down => {
-    const h = arrowHeight + 1
-    const svg = createSVGElement('svg')
-    svg.setAttribute('aria-hidden', 'true')
-    svg.setAttribute('width', arrowWidth)
-    svg.setAttribute('height', arrowHeight)
-    const polygon = createSVGElement('polygon')
-    polygon.setAttribute('points', down
-        ? `0 ${h}, ${arrowWidth / 2} 0, ${arrowWidth} ${h}`
-        : `0 0, ${arrowWidth / 2} ${h}, ${arrowWidth} 0`)
-    svg.classList.add(down ? 'popover-arrow-down' : 'popover-arrow-up')
-    svg.append(polygon)
-    return svg
-}
-
-export const createPopover = (width, height, { x, y }, dir) => {
-    const down = dir === 'down'
-    const fullHeight = height + arrowHeight
-    const overlay = document.createElement('div')
-    Object.assign(overlay.style, {
-        position: 'absolute',
-        left: '0',
-        top: '0',
-        width: '100vw',
-        height: '100vh',
-    })
-    const arrow = createArrow(down)
-    const top = clamp(0, window.innerHeight - (down ? height : fullHeight),
-        down ? y + arrowHeight : y - fullHeight)
-    Object.assign(arrow.style, {
-        position: 'absolute',
-        left: `${clamp(radius, window.innerWidth - arrowWidth - radius,
-            x - arrowWidth / 2)}px`,
-        top: `${down ? top - arrowHeight : top + height}px`,
-    })
-    const popover = document.createElement('div')
-    popover.setAttribute('role', 'dialog')
-    popover.classList.add('popover')
-    Object.assign(popover.style, {
-        position: 'absolute',
-        boxSizing: 'border-box',
-        overflow: 'hidden',
-        left: `${clamp(0, window.innerWidth - width, x - width / 2)}px`,
-        top: `${top}px`,
-        width: `${width}px`,
-        height: `${height}px`,
-    })
-    overlay.addEventListener('click', () => {
-        overlay.parentNode.removeChild(overlay)
-        arrow.parentNode.removeChild(arrow)
-        popover.parentNode.removeChild(popover)
-    })
-    return { popover, arrow, overlay }
-}