Issue #5109

Do not show the Wallet tab when running -disablewallet.
Also needs to take into account the build-time setting ie
```
#ifdef ENABLE_WALLET
bool enableWallet = false;
#else
bool enableWallet = !GetBoolArg("-disablewallet", false);
#endif
```
However this would duplicate the logic from bitcoingui.cpp. I'd prefer a slightly different solution: 
- make enableWallet an attribute of `BitcoinGUI` class
- pass it into the OptionsDialog constructor when constructed in `BitcoinGUI::optionsClicked`
- use the passed-in enableWallet here