On-Screen Gameplay Timer

[UTDZac]

This PR adds a simple timer to the game screen. This is a feature request from Issue #376 .

Please don't merge this PR in until the Resources PR has been merged in; it uses the required resource code changes.

Feature Notes

• The Timer can be turned on or off in the Extras Settings screen
• The Timer always appears in the bottom-right of the game screen itself, overlayed on top of the game itself
  • The time is formatted as HH:MM:SS, but omits the HH if play time is under 60 minutes
  • In a future patch, I will likely offer more options for relocating the timer
• The Timer can be paused/unpaused by clicking on it.
  • A help tip is displayed when the timer is first enabled to explain this
• The play time is tracked as a Tracker.Data.playtime stat, which is saved as part of the tracked data
• The play time also displays on the Stats screen and Game Over screen
  • The Timer is paused on the Game Over screen, and unpaused when the player clicks Continue or Retry Battle.
• For ease of implementation, the Timer's time tracking isn't very granular; it simply counts seconds-elapsed using os.time()
  • Because it uses os.time(), it's possible this doesn't work for Linux or Mac OS. I removed my VMs, so I wasn't able to test.