Add joinstr context sub menu menu in UTXO tab

The user should be able to right click on a UTXO and select 'create pool' or 'join pool'. This would automatically register a new bitcoin address and create or join the pool. The warning added in #22 should work here if selecting a UTXO with incorrect denomination for joining the pool.

In this case the UTXO selection tab should not ask the user to select input again and it should be done automatically based on previous selection.