Avoid unnecessary PlatformMessage copies in plugin callback loop

[pas2k]

Reworked PlatformMessage propagation to use pointers.

All golang values are passed by value. That means that when you pass a struct to a function, that's about as efficient as passing all of the struct members as parameters, including copying all the
nested strings.

This might be okay, or even beneficial from GC perspective for small structs such as PointerEvent, but is a really bad idea when used in loops (such as callback loop made by the plugin system).

The only reason you don't see much of pointers in golang stdlib is, it primarily operates on interfaces, while the interfaces themselves are mostly implemented on pointer, not value types.

Also reworked access to PlatformMessage.Message into pointers, for the same reason. No need to change the member itself into pointer -- it's only meaningful to pass with the corresponding PlatformMessage.

[pchampio]

@pas2k,
You pointed out a lot of mistake/slip made.
The work, comments and PR you made over the last week were always top notch.
I am giving you commit access to the repo 🥇

[pas2k]

@Drakirus, thanks! I'll try to do my best!