fix bug on request if transfert amount is 0

self.close() replaced with super().accept()
QErrorMessage replaced with QMessageBox.critical()

- Managing lost communities (when no nodes could be joined)
- Fixing broadcast management

Fixing caching bug with blocks being refreshed only every 5 minutes, even if the blocks changed.

New dialog created : PasswordDialogAsker
Needs to be passed throught the Widgets managing the current account

Fix bug contact radio button on transfer money dialog

Account now has Wallets and Communities
Wallets are now managing the network side of ucoin
A community doesnt depend anymore on a wallet
A wallet knows what community it is part of thanks to currency name