Add various path utility functions

[JDeeth]

This provides four new functions:

system_path()
prefs_path()
plugin_path()
aircraft_path()

They return PathBuf objects, which the user will presumably find straightforward to navigate from to get, e.g., the aircraft root folder.

I've moved the path_init function to the internal module and promoted paths to pub mod - is that OK?

[JDeeth]

I should mention, I've tested this on Windows only, but would hope it works seamlessly on other platforms!

Fixes #25

[mkreu]

Thanks for the contribution! Looks good to me, just have to minor comments

I will test it on linux and I guess we just hope it works on mac as well :D

[mkreu]

Suggested change

	pub fn path_init() {
	fn path_init() {

Unless I am missing something, there should be no reason anymore for this to be public

[mkreu]

We should add docs to the path module that we always return native paths because we turn on the resprective feature during xplm_init.

[JDeeth]

Sorry, I missed this... these functions return PathBuf objects from the Rust standard library - isn't that platform-agnostic? I thought the "native paths" feature was a convenience for C programmers!