Add a "Select all" button to BOINC Manager

Can we have a "Select all" button on the Projects, Tasks and Transfer tabs? It's already there, of sorts. Just click the first item you want to select, hold down Shift, click the last item you want in the list. Been there since BOINC 6.8

Rom: We should be able to do this. I had thought we had done that already, but it didn't do anything.

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