This patch introduces a Compile Kernels operation. Invoking
this operation will compile all viewport kernels of cycles.

It was used to check the validity of such an operation but came
to the next issues:

• Worst case the process took 1h.30m (serial compilation)
• At some point (on my system at around kernel 100) no new processes could be started (system() fails). Restarting the operation will compile some kernels more but will lead to this issue much faster. Only after restarting blender I was able to compile kernels again.
• Ideally the compilation of all kernels took 30m. Even this time is too much to make this operation really useful.
• Needed a lot of memory.

Please note that this is a prototype and not intended to be used in its current form.