obj_control: Translate a comment to English (from Czech)
obj_control: Fix global self pointer
obj_control: Remove player map (unused)
Add controller (gamepad) icon sprite
Separate icon sprites in `icon` group
obj_wall: Increase depth to 1 so walls don't cover players' arrows
Control players using special `obj_controller` objects

An `obj_controller` instance represents a controller, e.g. a gamepad.
A controller is held by at most one user (human player) at any time.
User can join or leave the game by pressing the start button on their controller.
Once in a game, a user can control their character using left and right buttons.
Swap positions of the players

Now they appear in sides of the screen that correspond to the position of their controls on the keyboard.
Randomize horizontal position of spawning player
Move origin of player sprite to the bottom
Add a third player

The third player is controlled by JKL and has blue colour.