Fix proof-of-stake block signing.

Add back mistakenly removed SignBlock() in proof-of-stake section of
BitcoinMiner().
Better fix for minting icon lagging the GUI.

Split updating user and network weights into a separate function. Update
them only every 30 seconds.
Sort transactions by date instead of status.

List transactions by most recent date on the overview and transactions
pages. Sorting by status shows all the orphaned blocks first, which is
confusing.
Fix the "For minting only" option.

When the wallet is unlocked with the minting only option, force the user
to retype the passphrase when sending coins by first re-locking the
wallet, then asking them for the passphrase to unlock it again.

Also changed the wording from "For staking only" to "For minting only".
Remove unneeded commented code.
Merge remote-tracking branch 'upstream/master'
Merge pull request #20 from skwurp/master

Fix the "For minting only" option.