Moved widget source code into seperate subdirectory

 - Core code is in "core" subdirectory
 - Widget code is in "widgets" subdirectory
 - Include paths "core/include" and "widgets/include" get merged into
   "build/include"

 - Adjusted runvalgrind script to get working directory correct
 - Added "pack.sh" script that generates a final, downloadable binary
   package