HP5590: scanbd integration is ready.

• Added read-only parameter --button-pressed for getting ID of last button pressed.
• Example usage:
  • Read last button pressed in the shell: "scanimage -d hp5590:libusb:... -h | grep button-pressed"
  • Integrate to scanbd by adding actions with
    • device filter = "^hp5590.*" and action filter = "^button-pressed.*"
    • using string-trigger with from-value = "none"
    • and to-value = "scan", "collect", "file", "email", "copy"
• Code extension uses code snippets by Cesello.
• White space fixes. Adjusted indents and replaced tabs by spaces for better formatting.