tweaks needed for the preferences tabs

For the sake of clarity I propose you make the following changes to the BOINC preferences screens:
(1) separate the CPU and the GPU preferences so that each has a different tab.
(2) add more options for the GPU settings; seems a little finer granularity is needed
(3) add specific controls for CUDA; give it options to switch tasks, not accept certain tasks, set a percent of the GPU it can use while using the graphic card for other apps, etc.
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