Add a GTK+ patch providing a "popup-style" combo-box style
property, allowing to manually control the combo-box popup-style.
Use the LIST style for all combo-boxes in our system-wide gtkrc
file; this only has effect if GTK+ was built with the above patch.
Add a GTK+ patch that makes sure that list-style combo-box popups
are never narrower than their content. This fixes the popup width
when using the System theme on Windows, for combo-boxes whose popup
cell-layout is wider than the combo-box cell-layout. It is also
required for the next commit.
