Partially addresses #518 , not sure where I can insert a config to modify color of terminal seperator so I'll put that one on hold for now.

1. Make it such that if handle_size is out of range, handle_size will be defaulted to smallest.
2. Terminator seperator size in gui settings will have range from 1 to 20, instead of -1 to 20 which is extremely misleading as -1 and 0 is out of range and previously defaults to a larger value.