Small fix to styles, workaround for one issue, one new option

The `overflow: hidden;` CSS accounts for content of a collapsed panel showing "under" the open panel in "SINGLE" mode. The `if` condition in the `collapse()` and `expand()` methods is a workaround for an issue I noticed where the `innerItems` were empty - most likely because the view was created, but not added to the Viewport yet.

The biggest change is a new option: `toggleOnTitlebar` which adds a tap handler to the entire titlebar for each accordion panel and allows the user to click/tap anywhere on the titlebar to toggle the accordion panel.