gioui.org/ui was renamed from gioui.org/ui/ui before the release,
but the import order wasn't changed accordingly.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
Gio doesn't support multiple windows anyway, so get rid of the
app.Windows channel and use NewWindow call for the mobile platforms
as well.
Signed-off-by: Elias Naur <mail@eliasnaur.com>