This release(0.2) adds save states and a crappy dialog window to change keys.
Unfortunately save states currently only work original GameBoy games, when loading the color ones, a portion of the VRAM seems to get screwed up.
Here's the hastily made dialog box, I'll add mouse and joystick/gamepad support later. Hopefully my old Gravis GamePad(it looks like an SNES controller) with its ancient DB15 connector will work.
I guess you're using Keys.ToString() for the text labels on the editor. Unfortunately this results in bizarre key names on anything other than a US layout; I hacked together some code that uses Win32 to convert Keys to a correct key name string, which you're more than welcome to pinch from here.