game_state: add user-controlled in-game timers

[jorgegv]

Allow definition of game timers:

• 16 bit counters
• They count seconds
• They have a name (for DATAGEN)
• They can be RESET, STARTed and STOPped
• They can be checked (FLOW checks)
• They can be acted upon (FLOW actions)
• There is always at least one global game timer (GAME_TIME) - This is to be implemented separately in #98
• Not managed by IM2 ISR, but from the main game loop - check for current_time.seconds changes

Tasks:

• [ ] New data struct timer_data_s: counter, state and initial state (running, stopped)
• [ ] New global table all_timers
• [ ] New functions: timer_init (called at program initialization), timer_reset (called on game initialization and in FLOW action), timer_start (called at game start and in flow action), timer_stop (called in flow action), timer_update_all_timers (called from main game loop), timer_reset_all_timers (called on game initialization)
• [ ] FLOW checks: TIMER_EQUAL, TIMER_MORE_THAN, TIMER_LESS_THAN
• [ ] FLOW actions: TIMER_STOP, TIMER_START, TIMER_RESET
• [ ] DATAGEN: directive for TIMER in GAME_CONFIG: name, initial_state