World upkeep logic

[PabloMansanet]

I imagine the game will roughly proceed in alternating phases, spaced by an even number of frames:

• A phase where all players decide and commit their actions, and these actions take effect on the world.
• A phase where all passive world effects trigger (bombs ticking down or exploding, points being granted, etc).

We'll need bevy systems to perform all of these "world upkeep" tasks. Some examples:

• Ticking down the bombs and/or exploding them.
• Increasing the point value of players on the hill.
• Cleaning up dead players, adjusting their point scores, and respawning them.
• Removing the breakable tiles that got hit by an explosion.

[PabloMansanet]

Picking this one up for now, though bombs won't be animated too nicely just yet :)

[PabloMansanet]

Finally implemented as a combination of #26 and #19.