Open a messagebox for better alerting user

add about menu to open the about popup window

Show members in table with columns for pubkey, uid, join date and expiration
Display members who will expire in 15 days as red

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

Added threading of account loading and blockchain watching so that the UI doesn't freeze anymore

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

Added the possibility to define a default account

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

Renaming actions in the member tab
Added a BlockchainInspector thread to check for new blocs and refresh the UI

Removed old files

Fixed a bug when an inputs would be removed from available ones even if the transaction is refused
Added icons
Display community nodes again
Save community nodes and load them when acccessing a community object

Signing on the way. There is a bug becuse of ucoin API using keyword "self", and it's conflicting with python self keyword.