Update en_US.ini

Delete no longer used line "Developer Tools" in [Develop]
Add new string: "File is a MODE2 image. Are you trying to open a PSX game?" I'll shorten it later.
"Debug" Menu in Win32 UI use shortcut "&D"
Add "&" for frame skipping -> 1
fix
Merge pull request #166 from wuspring/patch-1

Update en_US.ini