GTK: make explicit text controls selectable.

This doesn't apply to every GtkLabel I instantiate: only the ones
constructed as part of implementing the cross-platform CTRL_TEXT.
Those labels contain information that the dialog box is deliberately
communicating to the user, so it seems a sensible idea to make sure
they can be copy-pasted.

By default, this also seems to cause them to become able to take the
input focus, so I've reverted that. You can select them with the
mouse, but I think having them appear in the tab order is an
awkwardness too far, since they're not active in any other way.

unix/dialog.c

@@ -2506,6 +2506,8 @@ GtkWidget *layout_ctrls(
              */
             uc->text = w = gtk_label_new(uc->ctrl->label);
 #endif
+            gtk_label_set_selectable(GTK_LABEL(w), true);
+            gtk_widget_set_can_focus(w, false);
             align_label_left(GTK_LABEL(w));
             gtk_label_set_line_wrap(GTK_LABEL(w), true);
             break;