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/node/context.h

@@ -20,7 +20,7 @@ class BanMan;
 class BaseIndex;
 class CBlockPolicyEstimator;
 class CConnman;
-class CMainSignals;
+class ValidationSignals;
 class CScheduler;
 class CTxMemPool;
 class ChainstateManager;
@@ -74,7 +74,7 @@ struct NodeContext {
     //! Issues blocking calls about sync status, errors and warnings
     std::unique_ptr<KernelNotifications> notifications;
     //! Issues calls about blocks and transactions
-    std::unique_ptr<CMainSignals> validation_signals;
+    std::unique_ptr<ValidationSignals> validation_signals;
     std::atomic<int> exit_status{EXIT_SUCCESS};
 
     //! Declare default constructor and destructor that are not inline, so code