Params: validations

[zverok]

Action should have method for parameter validation, and call it automatically before perform. Possible validations:

• parameter types;
• parameter limitations (like "50 or less" for numerical params), including complex ones, like "50 for users, 500 for bots" (and MediaWiktory user can explicitly say "my code has bots right")
• forcing required params and param combinations, including complex ones (param A or B is required; param A and B are required; param A is required if param B is present, and prohibited otherwise; )