Build LoadingWindow_Gtk as an object module

CMake object modules have the advantage of allowing to set additional
compile flags for the involved files, but it's linked into the main
binary with the rest of the object files.

This removes GtkModule.so and the dynamic loading code that goes with
it.

Utils/install-stepmania.pl

@@ -240,7 +240,3 @@ $FoundSection || print "warning: SectionEnd not found\n";
 close F;
 
 system("cp -vp \"" . $bin_path . "stepmania5\" \"$instdir\"");
-if( -e $bin_path . "GtkModule.so" )
-{
-	system("cp -vp \"" . $bin_path . "GtkModule.so\" \"$instdir\"");
-}