Reorganize folder/build structure

--HG--
rename : plugin/chartsplugin.cpp => src/chartsplugin.cpp
rename : plugin/chartsplugin.h => src/chartsplugin.h
rename : plugin/piechart.cpp => src/piechart.cpp
rename : plugin/piechart.h => src/piechart.h
rename : plugin/pieslice.cpp => src/pieslice.cpp
rename : plugin/pieslice.h => src/pieslice.h
rename : plugin/qmldir => src/qmldir