Some property need to be known at compile time. We already had checks
that the binding is a compile time constant, but there was no check to
prevent, say
self.row = 42;
which wouldn't work or could even cause panic or miscompilation of
generated code
Closes#4037
This patch adds a `close()` function that can be called to close a popup
window, and a `close-to-click` boolean that can be set to false to
disable the default behavior.
