And also move private function to a private header.

We can't just call GtkWidgetClass::get_request_mode() anymore. If the
widget has a layout manager, we need to ask that one.

We don't do that in GtkWidgetClass::measure() implementations either, so
make the semantics match.

tbaederr authored May 24, 2019 and Timm Bäder committed May 27, 2019

This is GtkFileChooserEntry API, not GtkFileChooser API.

The modules don't depend on the default display
at all, and loading them more than once makes
the displayclose test fail here, locally.