Feature/53/add clock

[Lucas-2306]

Fixes #53

Feature: Added a timer (clock) that keeps track of the duration of the curren run.

Datetime imported

Added variables:

• TIMER_COLOR: stores the color of the timer shown in the screen
• time_start: stores the time at the beginning of each run
• time_delta: store the time span from the start of the run until the present moment
• time_render: stores the timer to be shown on the screen

When the "if not self.alive event" of the object "snake" returns "true", the value of the "time_start" is updated.