Add macro creation/edition for host prototypes

Having the capability to create user-defined macros when defining a host prototype can be really useful in order pass values to this host that where only available in the LLD context, like {#SNMPINDEX} or {#SNMPVALUE}.

Example : by creating a user-defined macros with the values of {#SNMPINDEX} on an host prototypes with a template attached, one can implement a LLD discovering sub-parts of an SNMP-enabled host (Power Supplys, Blades on a chassis, etc...) which then creates full-fledged hosts representing these sub-parts of the original host.
Then these hosts can have items requesting OIDs relevant to them, by using the attached template with items refering to the user-defined macros storing the {#SNMPINDEX} found during the LLD.