Enable intltool for localization of .desktop and other non-compiled files

This modification have following influences:
* Build instructions have to be updated - just run bootstrap.sh before doing ./configure
* The synfigstudio.pot is not generated at build time. To generate it and update translations use command "cd po && make update-po".
* Available languages are stored in the po/LINGUAS now. Don't forget to update instructions here - http://wiki.synfig.org/wiki/Dev:Translation
Enable localization for plugins