Rename staking field to reduce confusion?

[nathanielcwm]

Links to #847

Currently among new users there seems to be confusion around the staking line on the main page of the wallet.

I propose having it renamed to smth like: On Cooldown.

[jamescowens]

We will have to do some thinking on this. Stake doesn't mean on cooldown. It actually means "not spendable", which is 110 blocks after the stake, and is different than the 16 hour cooldown requirement. (Just to make things more confusing...)

[cyrossignol]

The precise term is "immature". If we need more clarity, we can label the field as "Immature (staked)" with a tool tip that reads, "Amount staked for a recent block that must wait for 110 confirmations to mature before you can spend it."

It isn't perfectly accurate because the field aggregates amounts for all immature UTXOs, but I think this tool tip is less confusing for most users than an alternative that tries to express those details. A majority won't stake another block before the previous stake matures.