[PL-78] - Fix issue

[kzryzstof]

This pull request adds support for tracking and displaying "measured progress" (such as "5/10") for achievements, improves the handling of achievement unlock timestamps, and refines the achievement display cycle to ensure the UI is never blank at session start. It also enhances session and achievement state management to avoid race conditions and stale data.

Achievement structure and progress tracking:

• Added a measured_progress field to achievement_t and retro_achievement_t structs, along with support for copying, freeing, and parsing this field from JSON messages. This allows achievements with partial progress to display their current state (e.g., "5/10"). [1] [2] [3] [4] [5] [6] [7]

• Updated the JSON protocol and documentation to include measured_progress and unlock_time for achievements. [1] [2]

Achievement unlock timestamp improvements:

• Improved handling of achievement unlock timestamps by using the real unlock_time when available, or a sentinel value when not, ensuring accurate and consistent unlock tracking. [1] [2]

Achievement display cycle and UI experience:

• Enhanced the achievement display cycle so that if there are no unlocked achievements, a random locked achievement is shown immediately, preventing the display from being blank at session start. [1] [2]

• Ensured proper freeing of the last unlocked achievement when the game changes, preventing memory leaks and dangling pointers.

Session and achievement state management:

• Refined the order of notifications and state changes when games or connections change, preventing race conditions where UI could be blanked out or left in a stale state. This includes firing notify_game_played and notify_active_identity before clearing achievements, and only firing notify_session_ready when a game is active. [1] [2]