When viewing a deeply nested path, there may be very little of the
PathBreadcrumbbar itself visible to double-click on. This solves that
by allowing double-clicks on its segment buttons to behave the same.
(With the caveat that it first selects the double-clicked segment.)
In order to make this work, `on_double_click` now takes the modifiers
instead of the MouseEvent. In this case we don't use it so that's fine,
but maybe we should make all mouse callbacks consistently take the
MouseEvent& as a parameter.
This Widget wraps both a Breadcrumbbar and a TextBox for editing the
path manually, based heavily on the existing code in FileManager.
Breadcrumbbar itself requires outside code to micro-manage what it does.
This class provides a simpler interface for it: Users don't have to
worry about segments, they just give/receive a string for the current
path.
