Move thread pool into true singleton

Thread pool needs to be stored in a single location for all of KiCad
otherwise each kiface will spin up its own pool of persistent threads

qa/tests/common/test_module.cpp

@@ -38,10 +38,7 @@ bool init_unit_test()
     bool ok = wxInitialize();
 
     if( ok )
-    {
-        // need these for library image functions
-        wxInitAllImageHandlers();
-    }
+        Pgm().InitPgm( true, true, true );
 
     return ok;
 }