UX: Don't allow user to open a channel with unconfirmed balance

[Kixunil]

Steps to reproduce:

• Make sure you have empty wallet (no funds)
• Create a transaction to fund the wallet (use low fee to ensure it doesn't confirm too early)
• Before the transaction confirms, try to create a new channel with the balance lower than the incoming transaction

What happens:

The wallet attempts to create the channel and fails because the transaction is unconfirmed

Expected behavior:

"Not enough funds" error just like when one attempts to create a channel with more than his balance. Or maybe "Not enough confirmed funds" or some similar message that makes it clear what's the problem.

Version: 0.3.0