Set the paste preview text in GTK4 to monospace (#4227)

A simple change to make the text preview in the paste confirmation
dialog monospace; this feels like something that most users would want,
or at least very few users would dislike.

We just call `gtk_text_view_set_monospace` and let GTK use whatever
default monospace font it wants to use. Theoretically we could probably
tell it to use whatever font the user has configured, but this should be
sufficient for most users.

src/apprt/gtk/ClipboardConfirmationWindow.zig

@@ -131,6 +131,7 @@ const PrimaryView = struct {
         c.gtk_text_view_set_bottom_margin(@ptrCast(text), 8);
         c.gtk_text_view_set_left_margin(@ptrCast(text), 8);
         c.gtk_text_view_set_right_margin(@ptrCast(text), 8);
+        c.gtk_text_view_set_monospace(@ptrCast(text), 1);
 
         return .{ .root = view.root, .text = @ptrCast(text) };
     }