X11 Copy and Paste does not work

The copy and paste feature of the X11 demo does not work. The code (found in nuklear_xlib.h) that accesses the X11 clipboard seems to have all come from one commit ([\#371](https://github.com/vurtun/nuklear/pull/371/commits/0073110c9b17b07bb78dd0f5c16778d7f5425d7f)). Since then, there have been no substantial changes to the respective lines of code.

Copy and paste in the gdi equivalent works (nuklear_gdi.h), so the problem seems specific to the xlib implementation.


Start in nuklear_xlib.h and compare its X11 clipboard handling with the working implementation in nuklear_gdi.h. Reproduce the problem in the X11 demo, then verify that copying and pasting work there without breaking the existing GDI behavior.

Written by the indexing model from the issue text.