Add Bash Shell support (partially fixes #44)

[xiaoyinl]

This PR adds a "Start Bash Shell" to menu, which opens Bash on Windows, if it's installed.

While I totally agree with the comments in #44 that it is better to make this configurable, I don't have enough Windows GUI knowledge to write a configuration dialog. So far I only implemented a menu item "Start Bash Shell", which launches bash.exe if it exists. I want to share this code so that, if you want, you can develop on top of this PR.

[NazmusLabs]

I'd love this to be included in the interm until a configuration system is developed. Having three most used shells isn't a bad idea.

I'm, curious though, if this throws an exception in older versions of Windows that predates WSL

[xiaoyinl]

@NazmusLabs It won't throw an exception if WSL doesn't exist. It just won't do anything if it can't find bash.exe.

[NazmusLabs]

@xiaoyinl Yeah, I realized that after I checked the code. :)

I did see that you did hard code the system32 folder so that the 32 bit version also works. I'm curious how this affects OSes prior to XP 64 bit, which I don't believe knows what sysdrive is, but I could be wrong.

[xiaoyinl]

@NazmusLabs I am not sure if it understands it or not. But that doesn't matter in this case, because if it doesn't understand, PathFileExists will return false, which is correct anyway, because Bash only runs on Windows 10 x64.