v0.8 - Config GUI

Merged PR #1

• Add GUI for editing ~/.config/rustgrab/config.json. Made with fltk-rs.
• Add ability to write to the config file.
• Remove dependency egui
• Set the content type when uploading files to Google Cloud Storage and XBackbone.
• Breaking Change
  • Change Type of default_screenshot_type in UserConfig to LScreenshotType