Bitcoin-Qt: move num TX label to debug window

- Bitcoin-Qt: remove num transactions from overviewpage 

- Bitcoin-Qt: add wallet category and num unique tx IDs in dbg window

-- this adds a new Wallet category to the debug window and implements a
  label which contains the current unique transaction ID count from the
  wallet (wallet model access is added to RPCConsole class)
-- layout was converted from table to form and a spacer removed
-- I also added an initialisation of the model variables in RPCConsole
  class, these were missing before

I had the idea because of issue #2175 and it also seems nice to be able to access walletmodel from RPCConsole for further additions to the debug window.