Clipboard

Hi, i have the clipboard working but i have some issues i would like to understand better. First question is @jadahl do i really need to get the serial from all input to use it to set a selection?
And_glfwPlatformGetClipboardString _glfwPlatformSetClipboardString are not thread safe and i can't find anywhere in the documentation if they should be or not.
BTW @jadahl can you review the patch?