[Feature Request] make location of jexiftoolgui_data configurable

[andrewcrook]

Problem Some user will want to move the location of the folder jexiftoolgui_data

Solution Add an environment variable and/or other configurable storage source to provide a path the override the default location.

[hvdwolf]

More or less a duplicate of #79.

I will have a look at it. Currently I am on low profile with my tool and don't spend hardly any time on it.