(hopefully) fixed issue when stoping while playing (wrong destruction order)

new facilities for foreground and background tasks with progress indication
initial setup and workspace loading is now indicated.

fixed some compilation issues under windows (mingw-w64 + qt 4.8)
removed QString::toStdString calls and added some missing includes

refactoring of skin parts.. check wether a skin is present, else: provide download
added auto skin downloading / extraction
project now requires qt's network support :/ better solution?

changed project file to include 7z library (.c files, single-threaded) within compilation