[WIP] Changes for CPlugin ABI v2 breaking changes

[CromFr]

This issue will list useful changes that will be bundled into the next NWNX4 version that needs to break the CPlugin ABI (and increments nwnxcplugin_abi_version)

NWNXCPlugin_InitInfo argument

Consider if it is necessary / useful.

NWNXCPlugin_New(NWNXCPlugin_InitInfo info);
// =>
NWNXCPlugin_New(const NWNXCPlugin_InitInfo* info);

Reorder NWNXCPlugin_InitInfo content

The current order has been built by appending new attributes at the end of the struct to improve compatibility.

Avoid strcpy for NWNXCPlugin_GetString return value

Currently NWNX4 provides a dest buffer to NWNXCPlugin_GetString so that the plugin can write its returned value into.

However in some cases the plugin already stores a null terminated string, and should be allowed to directly return this string, as it's always strcpy-ed into a NWScript string (need to double check this, just to be sure).

NWNXCPlugin_GetString should return a const char* in order to return a null terminated string. The plugin will be able to chose between returning the buffer provided as parameter, or return a custom string that's already allocated.

Note: Another solution is to give a char** result parameter, and have the plugin *result = "hello world"; or sprintf(*result, "...", ...)

Allow to switch CNWSMessage for ExecuteScript

See issue #80