X11: Add native access to primary selection

This adds the native access functions glfwSetX11SelectionString and
glfwGetX11SelectionString under GLFW_EXPOSE_NATIVE_X11.  They are
similar to glfwSetClipboardString and glfwGetClipboardString but operate
on the PRIMARY selection.

The primary selection is widely used in X11, and so seems important to
support.  Primary selection is mostly an X11-specific thing, hence it's
exposed as an X11 native interface.

Fixes #894.
Closes #1056.

Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

src/x11_platform.h

@@ -161,6 +161,8 @@ typedef struct _GLFWlibraryX11
     XIM             im;
     // Most recent error code received by X error handler
     int             errorCode;
+    // Primary selection string (while the primary selection is owned)
+    char*           primarySelectionString;
     // Clipboard string (while the selection is owned)
     char*           clipboardString;
     // Key name string
@@ -214,6 +216,7 @@ typedef struct _GLFWlibraryX11
     Atom            TARGETS;
     Atom            MULTIPLE;
     Atom            CLIPBOARD;
+    Atom            PRIMARY;
     Atom            CLIPBOARD_MANAGER;
     Atom            SAVE_TARGETS;
     Atom            NULL_;