Web apps in the left panel were a fixed 380 px, too narrow for WhatsApp,
and add-on panels a fixed 400. A grip on the panel's right edge now
resizes it, each app keeps its own width (settings.leftPanelWidths), a
double-click resets it, and the page always keeps at least 320 px. The
grip is a small view of Theseus's own laid over the edge, since the panel
may be a third-party page; while dragging it covers the window so the drag
keeps its events.
Like Opera's sidebar, the left panel now hides when a new tab is opened or
the right sidebar opens. It is only hidden, so reopening lands where the
user left off.
Add-on panels on the left used to get the right sidebar's grip on their
left edge, where dragging resized the right sidebar; that grip is hidden
there and its drags are ignored.
