release 1.10.6

- fixed backspace with undef fraction
- altered stack not redrawn after undo
- digits added/removed to/from integer part in correct number base
- allow modules to not load but without error
- display welcome at startup, also showing version and modules
- preaction hook; undo functionality moved to module
- only first module run of multiple with the same name
- invalid commands shown as strings instead of character codes