- add a few messages and warning in core
- no longer try to parse .otf files with paje parser when otf is not compiled
- fix a bug with vertical line not disappearing when clicking on containers
- memory improvements :
    - do not store extra_fields for each entity as an empty map when they are not present but as a null pointer (need more checks to avoid segfaults, hope I didn't forget some in drawtrace and other files needing get_extra_fields)
    - remove a virtual destructor in Entity.h (virtual functions add memory overhead to their inheriting objects)
-> on my windows, sizeof(Link) was 72, it is now 56, and for a 55Mb trace I went from 325 Mb with 1.2 to 275 Mb of used memory with trunk