Variable declaration improvement ideas

[xiaohh2000]

Variable declaration in OpenPLC is not user friendly, here are some improvement ideas

1. Support import from/export to CSV file, since edit CSV file is much easier.
2. Global variables defined in Res0 must be defined as external in other modules, to add all of them one by one for each module is time consuming. So I would suggest to ignore the external step, all global variables can be access from any other modules by default.
3. The variable declaration part locates at the top of each module, it would be better if this can be float or relocated by the user.