Provide toplevel build.xml

Changes:

   - creates binary/ and build/ inside the toplevel directory
   - fix clean target
   - fix forgotten deltree statement outside any target
Adjust make.sh to use toplevel build.xml
Drop old src/scripts/build.xml

Replaced by toplevel build.xml
Make plugins, extensions and lib directory relative to graphtea-main.jar
Load extensions from directory relative to graphtea-main.jar
Don't cd into binary/ anymore

With the latest changes, graphtea can be run from any directory, so it's
no longer necessary to cd into binary/.
Exclude Photoshop source files from builddir and distdir
Allow src/scripts/lib/ to be empty
Merge pull request #23 from adiknoth/proposed-master

Project cleanups