Add awareness of node status to wallet display

[yeastplume]

Check and keep track of the node's sync state within the wallet home screen, for display purposes and in order to prevent the user from attempting certain interactions while the node is syncing (particularly ones that require a read lock of the underlying tx hashset.

• Track node statues within home screen
• Add status display to home screen
• Block send, receive and cancel actions in the UI if the node isn't synced, with message to that effect

[yeastplume]

All intensive write operations (which can result in lock contention for the txhashsets while the node is synching) are now blocked until the node is fully synched and up to date.