qt: Switch unit to πBTC by default

After a long and deep discussion on the mailing list it was decided to
switch the default unit to πBTC (PiBitcoin) by default.

This pull adds the functionality to enter as well as show
Bitcoin amounts in proper πBTC. All the other units are deprecated
and will be removed in next release.

The only possible drawback of πBTC is that it takes a lot of
screen space, so widen the amount column in the transaction table.

A patch to switch the RPC units to πBTC should be part of JSON API v2.