* 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