Merge #13634: ui: Compile boost::signals2 only once

fa5ce27385 ui: Compile boost:signals2 only once (MarcoFalke)

Pull request description:

  ui is one of the modules that poison other modules with `boost/signals2` headers. This moves the include to the cpp file and uses a forward declaration in the header.

  Locally this speeds up the incremental build (building everything that uses the ui module) with gcc by ~5% for me. Gcc uses ~5% less memory.

  Would be nice if someone could verify the numbers roughly.

  I presume the improvements will be more pronounced if the other models would stop exposing the boost header as well.

Tree-SHA512: 078360eba330ddbca4268bd8552927eae242a239e18dfded25ec20be72650a68cd83af7ac160690249b943d33ae35d15df1313f1f60a0c28b9526853aa7d1e40

src/noui.cpp

@@ -52,7 +52,7 @@ static void noui_InitMessage(const std::string& message)
 void noui_connect()
 {
     // Connect bitcoind signal handlers
-    uiInterface.ThreadSafeMessageBox.connect(noui_ThreadSafeMessageBox);
-    uiInterface.ThreadSafeQuestion.connect(noui_ThreadSafeQuestion);
-    uiInterface.InitMessage.connect(noui_InitMessage);
+    uiInterface.ThreadSafeMessageBox_connect(noui_ThreadSafeMessageBox);
+    uiInterface.ThreadSafeQuestion_connect(noui_ThreadSafeQuestion);
+    uiInterface.InitMessage_connect(noui_InitMessage);
 }