glfw drag and drop

adds glfw support for drag & drop. most of the modifications are done to glfw itself. i'll send a PR to glfw and work with them to get this included in the official version

it has the same features as our hacked glut but it also works on linux now

@ofTheo, i've also removed your hack for windowGetPosition since that it's fixed too in the master branch of glfw, your solution was also giving some problems for me like the window was jumping a lot in the windowExample