Allow different custom templates for different Event Info / Registration and for Online Contribution Pages

1. Allow different custom template "instances" for the event info and event registration screens for a particular event on the basis of event id.

2. Allow different custom template "instances" for the online contribution page screens(Main, Confirm, Thankyou) for a particular contribution page instance on the basis of contribution page id.

EXAMPLES:
Custom templates for the 3 Online Contribution screens for Contribution Page with ID = 1 would be:

CRM/Contribute/Form/Contribution/1/Main.tpl
CRM/Contribute/Form/Contribution/1/Confirm.tpl
CRM/Contribute/Form/Contribution/1/Thankyou.tpl

Custom templates for the 3 Online Event Registration screens for an Event with ID = 3 would be:

CRM/Event/Form/Registration/3/Register.tpl
CRM/Event/Form/Registration/3/Confirm.tpl
CRM/Event/Form/Registration/3/Thankyou.tpl

... and to customize the Event Info screen for an Event with ID = 3:

CRM/Event/Page/3/EventInfo.tpl

(This feature is simliar to what has already been impelemented for profile on the basis of gid.)