Make events functions `OnEnabled()` and `OnEnabled()`
protected rather than public.
Move initialization and clean up logic into
`OnCreated()` and `OnDestroyed()` event functions.
This feature fixes several issues related to the selling price of both
single and dual pistols, all originating from the existence of dual weapons.
Most notable issue is the ability to "print" money by buying and
selling pistols in a certain way.
It fixes all of the issues by manually setting pistols'
'SellValue' variables to proper values.
