Split EmptyWalletWindow into BsqEmptyWalletWindow and BtcEmptyWalletWindow

[lusarz]

There is a lot of checks like:

if (!isBtc)
        gridPane.getColumnConstraints().remove(1);

if (isBtc)
        addMultilineLabel(gridPane, ++rowIndex, Res.get("emptyWalletWindow.info"), 0);

in EmptyWalletWindow

In this pull request I split this logic into separate classes, to make it more readable.

[ripcurlx]

ACK