improve/port Tk event handling

[JeffBezanson]

extras/tk.jl currently uses an X11-specific hack to handle events. We need to use a more portable method of integrating Tcl/Tk's event loop with our own.

We may need to use Tcl_CreateEventSource, as documented here: http://www.tcl.tk/man/tcl8.4/TclLib/Notifier.htm

This may require adding features like timers to our event loop, which will happen already with the libuv branch.

[JeffBezanson]

Issue moved to Tk package: JuliaGraphics/Tk.jl#3