Add frontend <-> wallet extension commands.
Add frontend <-> wallet extension commands.

This change adds support for the following extension commands:
- createencryptedwallet
- getbalances
- walletislocked

cqswxj2013/btcd,soapboxsys/ombfullnode,aakselrod/btcd,wallclockbuilder/btcd,jimmysong/btcd,dajohi/btcd,DanielKrawisz/btcd,Dirbaio/btcd,Cleawing/btcd,DanielKrawisz/btcd,wallclockbuilder/btcd,jcmincke/dcrd,soapboxsys/ombfullnode,ppcsuite/ppcd-old,Dirbaio/btcd,mathiasrw/btcd,1d4Nf6/btcd,jimmysong/btcd,tb00/btcd,jcmincke/dcrd,BitGo/prova,CryptocurrencyCabal/htcd,qchain/btcd,runeaune/btcd,FactomProject/btcws,jrick/dcrd,glv2/ppcd,Stash-Crypto/btcd,cjepson/btcd,jongillham/btcd,hectorj/btcd,ltcsuite/ltcd,pkothbauer/btcd,glv2/ppcd,jrick/btcd,toddfries/btcd,Stash-Crypto/btcd,toddfries/btcd,ppcsuite/ppcd,dan-da/btcd,cjepson/btcd,jcmincke/dcrd,cqswxj2013/btcd,dajohi/btcd,jrick/dcrd,adiabat/btcd,aakselrod/btcd,ltcsuite/ltcd,BitGo/prova,jrick/btcd,dajohi/btcd,qchain/btcd,1d4Nf6/btcd,tb00/btcd,Roasbeef/btcd,davecgh/btcd,btcsuite/btcd,ppcsuite/ppcd-old,decred/dcrd,btcsuite/btcd,stevenroose/btcd,btcsuitereleases/btcd,decred/dcrd,Dirbaio/btcd,dan-da/btcd,Cleawing/btcd,btcsuitereleases/btcd,ppcsuite/ppcd,danoctavian/btcd,pkothbauer/btcd,hectorj/btcd,jongillham/btcd,jrick/btcd,adiabat/btcd,stevenroose/btcd,mathiasrw/btcd,danoctavian/btcd,Roasbeef/btcd,CryptocurrencyCabal/htcd,runeaune/btcd,davecgh/btcd