FlaUI dll usage should be configurable

[Nepitwin]

Robotframework-FlaUI should contain a configuration paramter to decide which FlaUI dll should be used.

• Net Core 3.0
• Net Framework 3.5 / 4.0 / 4.5
• Custom path to specific FlaUI dll
• Extend library property as flaui usage.
• By default Net Framework 4.5 should be used.

Example usage

Library         FlaUILibrary  flaui=<core|framework_3_5|framework_4|framework_4_5|custom_path_to_dll>