Code refactoring and optimization

[Joe3n]

• Scan_interval can be customized
• Outsource all const variables to const.py
• Use homeassistant.const instead of custom defininitons
• Use predifined variables for device class and units
• Provide icons for particulate matter sensor
• Add unique_id for UI editing

[lichtteil]

Thank you a lot @JoachimEngelmann! I will have a look at it this week :)