Renames menu_entry to desktop_file

Currently menu_entry has three responsibilities
1) Parse a .desktop file
2) Transform it to representation suitable for a JWM menu entry
3) Create a JWM representation out of it

Task 2 would go to transform/jwm and task 3 would go to menu_entry/jwm

This would allow future extension to other menu systems as well with ease

The commit creates currently non-compiling skeleton for transform/jwm and menu_entry/jwm