Allow to configure different resource usage for CPU and GPU

[sirzooro]

Use case: I would like to use CPU to crunch tasks from PrimeGrid, and at the same time set it as a backup project for GPU. Now it is not possible, I have to keep PG in backup mode for CPU even if it is main project for it, or choose another project as a backup for GPU.

[Aurum420]

This can be easily handled by the project's preferences page.