"Created" date

[kirillt]

It seems that we have 3 different ways to provide created_at timestamp:

• extracted automatically for specific resource kinds, e.g. from EXIF
  must be stored in .ark/cache/metadata
  • Extract metadata from images ark-android#56
• specified manually by user or automatically by an app
  must be stored in .ark/user/properties
  • Extend PropertiesStorage with date field arklib-android#89
• generated automatically for any resource during its first indexing
  must extend .ark/index
  • "First discovered" attribute of resources #66

[kirillt]

This created_date attribute must be possible to use in resource sortings.