GTK.jl support

[Taaitaaiger]

I want to be able to use GTK.jl in combination with jlrs, in particular I want it to be possible to have multiple GR plots open and manipulate them without blocking the main thread. Since GTK.jl automatically schedules the GTK thread as a task on the main thread when the package is loaded, it must become possible to schedule tasks on the main thread from jlrs because using tasks running on another thread either causes a crash or hangs indefinitely.

In order to ensure these tasks are executed, control of the main thread must be yielded periodically by calling jl_yield.