Functionality+Blender UI for setting price on a scale

[indyjo]

The end user shouldn't need to type in monetary amounts when setting a price per tile in Blender. Instead, pricing should be chosen by the user on a scale:

• Economic
• Recommended
• Priority

Those prices should come from statistics that the service should generate for an each article/currency combination. Either a slider or a 3 to 7 button row seems approriate for setting price.

This issue comprises

• a service API to query statistics about prices of a specific article/currency combination
• BitWrk client logic to recommend prices based on these statistics
• a BitWrk-client API to receive these recommended prices
• Blender UI+logic to chose a price within the range as stated above

This does not yet include dynamic pricing, i.e. automatically adapting price with time, which would require the user to be able to at least state a maximum price per tile.

[indyjo]

See also discussion around #195