Adds DialogParams::un_options - A set of FOS_* flags to *unset* from system defaults

[rscarson]

• To allow override of default dialog behaviour
  • Example: FOS_FILEMUSTEXIST and FOS_OVERWRITEPROMPT cannot be unset/modified currently
• Fixes clippy warnings (un-needed referencing and is_empty checks, mostly)
• Cargo fmt ran automatically, I can revert the formatting fixes if you want

[ben-wallis]

Thanks for the PR! Version 0.1.8 is now published including this change.