[wallet] Support creating a blank wallet

A blank wallet is a wallet that has no keys, script or watch only things.
A new wallet flag indicating that it is blank will be set when the wallet
is blank. Once it is no longer blank (a seed has been generated, keys or
scripts imported, etc), the flag will be unset.

src/interfaces/wallet.cpp

@@ -463,6 +463,7 @@ public:
     }
     unsigned int getConfirmTarget() override { return m_wallet.m_confirm_target; }
     bool hdEnabled() override { return m_wallet.IsHDEnabled(); }
+    bool canGetAddresses() override { return m_wallet.CanGetAddresses(); }
     bool IsWalletFlagSet(uint64_t flag) override { return m_wallet.IsWalletFlagSet(flag); }
     OutputType getDefaultAddressType() override { return m_wallet.m_default_address_type; }
     OutputType getDefaultChangeType() override { return m_wallet.m_default_change_type; }

src/interfaces/wallet.h

@@ -235,6 +235,9 @@ public:
     // Return whether HD enabled.
     virtual bool hdEnabled() = 0;
 
+    // Return whether the wallet is blank.
+    virtual bool canGetAddresses() = 0;
+
     // check if a certain wallet flag is set.
     virtual bool IsWalletFlagSet(uint64_t flag) = 0;