scripted-diff: Rename MainSignals to ValidationSignals

-BEGIN VERIFY SCRIPT-
s() { git grep -l "$1" src | xargs sed -i "s/$1/$2/g"; }

s 'CMainSignals'    'ValidationSignals'
s 'MainSignalsImpl' 'ValidationSignalsImpl'
-END VERIFY SCRIPT-

src/init.cpp

@@ -1164,7 +1164,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }, std::chrono::minutes{5});
 
     assert(!node.validation_signals);
-    node.validation_signals = std::make_unique<CMainSignals>(scheduler);
+    node.validation_signals = std::make_unique<ValidationSignals>(scheduler);
     auto& validation_signals = *node.validation_signals;
 
     // Create client interfaces for wallets that are supposed to be loaded