Engine: don't crash on non existant audio files
[WIP] Improving the QLC+ Script function

- ScriptEditor: commented non-finished macros
- Added systemcommand, to hook up QLC+ with the execution of external programs/scripts
- Written totally missing documentation about Script/Script Editor
- improved setfixture channel selection

TODO:
- add random() macro
- add file selection macro
- check paths with spaces (especially on Windows). They should break the tokenizer method
- display a syntax error log when loading/modifying a script
- improve wait and setfixture macros using standard QLC+ dialogs/widgets
- evaluate the jump macro started by Heikki (even though I predict more disasters than benefits)
- add line numbers ?
[WIP] More Script function improvements

- added random macro
- added file path macro
- wait times use the QLC+ way for times
- updated/fixed documentation

TODO
now it's really necessary to have a check for
- spaces in file paths
- syntax errors