Includes: Cleanup around net main and wallet

-Move from .h to .cpp: in main, net and wallet
-Remove unnecessary #include "main.h"
-Cleanup some wallet files includes

src/qt/sendcoinsdialog.cpp

@@ -17,7 +17,7 @@
 
 #include "base58.h"
 #include "coincontrol.h"
-#include "main.h"
+#include "main.h" // mempool and minRelayTxFee
 #include "ui_interface.h"
 #include "txmempool.h"
 #include "wallet/wallet.h"