Make declarative definitions of payable functions

[ForNeVeR]

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Currently all payable functions of PetPlugin do checks of user account and removes the petcoins by itself. We should construct more centralized code for this process.

[ForNeVeR]

Done by 586f28ef67d14f0e668f937fef07ba292538549b.

[rexim]

Are you sure we can close this one? I didn't implement declarative definition of payable functions. They still do checks of user accounts and other stuff. I just slightly offloaded the PetPlugin class. :)

[ForNeVeR]

Well, you're right. We'll need some additional work here.

[ForNeVeR]

Rescheduling to 1.1. There's too many work here.