Fl_Theme_Chooer: Don't uselessly spin while theme window is open.

src/Fl_Theme_Chooser.cxx

@@ -236,5 +236,5 @@ void fl_theme_chooser() {
   w->show();
   
   while( w->shown() )
-    Fl::check();
+    Fl::wait();
 }

src/Fl_Theme_Chooser.fl

@@ -176,5 +176,5 @@ w->end();
 w->show();
 
 while( w->shown() )
-  Fl::check();} {}
+  Fl::wait();} {}
 }