Add GtkButton:can-shrink

Add GtkButton:can-shrink

For certain kinds of layouts, especially ones where one or both sizes of
a top level is constrained by physical limits, it's acceptable to have
buttons that rely on the minimum size of their contents, rather than the
natural size. It is left to the application authors, or the localization
teams, to ensure that things like translations and font sizes do not
result in a broken UI.