I reckon it would be a very difficult program to write and to debug. But if you feel like doing it can I request that you do it with a programming system that works on Linux and Mac as well as Windows?
Unfortunately, should I choose to have a crack at it, I don't have easy access to Linux and Mac computers plus software in order to do it.
The best I might be able to do is to try an separate the arduino simulator from the Windows UI stuff with some sort of standardized interface into the simulator. And that would theoretically make it easier for others to port it over to other operating systems.
Also I would really need to work with others who have a far more in depth knowledge of how the Arduino IDE architecture works. I am unlikely to succeed on my own with my very limited knowledge in this area. Or if I did it might take 20 years on my own. I strongly suspect that such an undertaking is not really a one man show.
On the up side Windows MFC makes it really easy to setup a basic text editor. In fact there some ready made example projects that could be adapted.
I have been working on an enhanced PString class (I have renamed it to CString) class.
You declare the variables like you do with PString but it contains most of the functionality of String, plus a bit extra.
I have had a bitch of a time trying to debug some of the functions with println statements and got sick to death of it. So I ported the class over to Visual Studio 2015 and debugged them there - much easier.
It did not take all that many code changes to get the class to compile in Visual Studio. I just set up a basic dialog box base app and called my string functions in an initialisation function (OnInitDialog(...)
Visual Studio 2015 Community is free if you want to try it.
All I had to do is swap my strcat functions for strcat_s, my strcpy functions for strcpy_s and make some typedefs e.g.
typedef uint8_t byte;
typefef uint32_t DWORD;
And that was about it to make my archuino class compile in Visual Studio.
If you don't use the types then you will have even less work because all the standard C types in arduino are the same in visual studio.