Linux copy/paste support

A number of changes here, I had to move the clipboard functions from unix_vid.c to unix_input.c since they should be with the XEvent handler.

GetClipboardData is unchanged and works. Some of my apps (sublime text) wont paste into qfusion, probably offering UTF8_STRING instead of XA_STRING, I might look into this.

SetClipboardData and FreeClipboardData have been filled out, and the X11 SelectionRequest events are handled. I mostly used [glfw](https://github.com/glfw/glfw/blob/master/src/x11_clipboard.c) and [x_clipboard](https://github.com/edrosten/x_clipboard) as references.

Finally, I hacked in ctrl-c/v input recognition with 815034b. There might be a better way to support the bind, but this works and it looks ugly when put in client/{console.c,keys.c}