Split gui and emulation core into seperate projects

Still needs a lot of work but this should serve as a ground for discussion in the meantime.

Basically it's split it up into `emucore` and `rpcs3`.    
* Emucore containing all non-GUI and non-WX related functionality.    
* Rpcs3 containing all GUI elements and wx platform stuff.

Since there's wx APIs used in the core, I temporarily made a shim layer between emucore and the used parts of wx that contains

* rConcurrency (things like wxSemaphore, wxCriticalSection, wxCriticalSectionLocker, etc. should be easily reproducable with C++ stdlib means like condition variables)
* rFile (wxFile, wxDir and various file and folder deletion, creation,etc, should be feasibly replaced with `#include <filesystem>` stuff or boost filesystem while it's not wide-spread among the compilers we use)
* rMessageBox (should probably be replaced by a gui overlay later on but for now a simplified API would suffice I guess)
* rPlatform (GLContext creation, wxImage to save an image and instantiation of platform dependant IO handlers like `WindowsKeyboardHandler` etc., this needs to be cleaned and expanded to take some load off of System.h)
* rTime (wxDateTime and related functionality, no clue what to replace this with yet)
* rXml (wxXml, no clue which xml API to use, probably something simple and self-contained that can be used in a header or two)