Steampak

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+ Added User.traverse_inventory() method.
+ Implemented `get_cards_stats` CLI command.
* Fixed market item price parsing (`decimal.InvalidOperation` error).

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+ Basic Steam Web API tools: User, Application, Market item, Card.
+ Added command line interface.
+ CLI: Get app cards
+ CLI: Get app card prices
+ CLI: Get market item price
+ CLI: Get user owned games
+ CLI: Get user gems count (requires public inventory)
+ CLI: Get price stats for booster packs owned by user (requires public inventory)

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+ Added simple demo app for debugging purposes.
+ Open overlay.
+ Open user profile.
+ Open user chat window.
+ Open user stats window.
+ Open user achievements window.
+ Open user add as a friend dialog.
+ Open user remove from friends dialog.
+ Open user accept friend invitation dialog.
+ Open user ignore friend invitation dialog.
+ Open group page.
+ Open/join group chat.
+ Save user stats (through achievements interface).

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+ Basic functionality.