* Qt5 compatibility
* src/net.cpp: fixed miniupnpc issue
Port from Bitcoin: https://github.com/laanwj/bitcoin/commit/61d85071405b99c3734606eed31ea8f615c0c77a#diff-772f489c7d0a32de3badbfbcb5fd200dR553
Fix MinGW build issue and simplify code.
Fix grayed out X-button (close) on Windows with Qt5.
Merge pull request #122 from phelixnmc/Qt5_compatibility

Qt5 compatibility
Start with UTXO database implementation.

Start with the implementation of a UTXO set database for tx
verification.  This is a squashed commit of the old utxo branch after
rebasing it onto the DatabaseSet changes.
Update the UTXO set in DisconnectInputs.

Also handle the UTXO set in DisconnectInputs.  Not yet really tested.
Skip unspendable outputs in UTXO set creation.

For certain types of unspendable outputs (network fee OP_RETURN scripts
as well as names that have been expired at the last checkpoint), don't
include the tx outputs in the UTXO set.
Make IsUnspendable accept the CTxOut directly instead of the script.