This avoids having to query the coinbase when creating the miner, which
in turn eliminates the dreaded startup error when no accounts are set
up. Later, this will also allow us to simply restart the miner when the
user picks a different coinbase.
This causes a lot of changes in other packages. These are included in
this commit because they're impossible to separate.
* Miner has been rewritten
* Added new miner pane
* Added option for local txs
* Added option to read from MergeMining contract and list them for
merged mining
