WebKit fires `dblclick` only for the second click of a burst. A quick
double-click right after another arrives as `click` with `detail` 3 and 4 and
no `dblclick`, so the second double-click on a folder in the Project view (and
on any tree row, tab or list item handled by `onDoubleClick`) did nothing.
A window-level shim now dispatches a `dblclick` on the click's target for every
later even click of a burst (detail 4, 6, ...), after the page's own click
handlers. Existing `onDoubleClick` handlers and their `stopPropagation` apply
unchanged; an engine that fires its own `dblclick` for that click gets it
dropped as a duplicate.
The Project view test now delivers bursts the way WebKit does (`dblclick` only
at detail 2) and checks both expand modes over 2–6 click bursts.
Refs #724
