qt: Plug many memory leaks

None of these are very serious, and are leaks in objects that are
created at most one time.

In most cases this means properly using the QObject parent hierarchy,
except for BanTablePriv/PeerTablePriv which are not QObject,
so use a std::unique_ptr instead.

src/qt/recentrequeststablemodel.cpp

@@ -14,7 +14,7 @@
 #include <boost/foreach.hpp>
 
 RecentRequestsTableModel::RecentRequestsTableModel(CWallet *wallet, WalletModel *parent) :
-    walletModel(parent)
+    QAbstractTableModel(parent), walletModel(parent)
 {
     Q_UNUSED(wallet);
     nReceiveRequestsMaxId = 0;