Add additional type checking etc. for py_cui functions so errors get caught on startup

[jwlodek]

Describe the bug Currently, something like a key_command not being valid will only be caught once the command is executed.

To Reproduce Steps to reproduce the behavior:

1. Create a py_cui application
2. Use the add_key_command function, and add a command that takes in 1 argument
3. Start the UI, and press the key
4. UI will crash.

Expected behavior An error should be spawned at startup when the function is called, for example whenever add_key_command is envoked with an invalid command function

Environment:

• OS: N/A
• Terminal: N/A
• Version: N/A