Make padding between popup and container configurable

Was hard-coded to 10px. Allow users to override in config.

Change-Id: Ib0b236d5ee59d2a55034c89a30ffed5e65739cca
Fix anchor positioning on popups

Broken for two reasons:
* $anchor.width() was always returning 0 for the pure CSS
  anchor. scrollWidth gives you half the actual width.
* Checking for closeness to edge was only based on alignment,
  not actual distance from edge, which can require adjustment even
  when align=center.

Change-Id: Ie7850fb98e545c7bac89a9179b2bb6527dcb1150
PopupWidget: Remove references to 'show' and 'hide' events

These were killed back in the dialog refactor in July (Id121fc3a), but the documentation remained.

Bug: 72168
Change-Id: I91c890110e0c049e9150d5be4c61404aacef2906
Merge "PopupWidget: Remove references to 'show' and 'hide' events"
Merge "Fix anchor positioning on popups"
Merge "Make padding between popup and container configurable"