Merge pull request #3787 from dogmaphobic/offlineMaps

src/QmlControls/OfflineMapButton.qml

src/QtLocationPlugin/QGCMapEngine.cpp

src/QtLocationPlugin/QGCMapEngine.h

src/QtLocationPlugin/QGCMapTileSet.cpp

src/QtLocationPlugin/QGCMapTileSet.h

src/QtLocationPlugin/QGCTileCacheWorker.cpp

src/QtLocationPlugin/QGCTileCacheWorker.h

src/QtLocationPlugin/QMLControl/OfflineMap.qml

src/QtLocationPlugin/QMLControl/QGCMapEngineManager.cc

src/QtLocationPlugin/QMLControl/QGCMapEngineManager.h