Add an accelerator (keyboard shortcut).
Flag is one of ACCEL_LOCKED, ACCEL_SIGNAL_VISIBLE and ACCEL_VISIBLE
The signal is the signal to invoke when the accelerator key is pressed.
The modifiers is a bitwise or of one or more of GDK.ShiftMask,
GDK.LockMask, GDK.ControlMask, GDK_MOD1_MASK, GDK_MOD2_MASK, GDK_MOD3_MASK, GDK_MOD4_MASK and GDK_MOD5_MASK.
The group is the accelerator group in which the accelerator should be added.
The key is the unicode code for the key to bind.