Moved core files from libs directory to src

CMakeLists.txt

src/CMakeLists.txt

src/core/Android/tandroid.cpp

src/core/Android/tandroid.h

src/core/Android/tfiledialog.cpp

src/core/Android/tfiledialog.h

src/core/CMakeLists.txt

src/core/changes

src/core/checktime.h

src/core/core.pro

src/core/core.qrc

src/core/exam/tattempt.cpp

src/core/exam/tattempt.h

src/core/exam/texam.cpp

src/core/exam/texam.h

src/core/exam/textrans.h

src/core/exam/tlevel.cpp

src/core/exam/tlevel.h

src/core/exam/tqagroup.cpp

src/core/exam/tqagroup.h