Add "ghosts" feature

[city41]

Just like in mario maker, enable the user to turn on "ghosts". where after they have tested their level, a trail of semi-transparent marios are overlayed to show where they went while testing. The purpose is to make it easier to figure out where to place things based on mario's capabilities.

this doesn't seem too hard, mario's x position is an unsigned 16 bit value stored in gba memory at 3003f20 and y is at 3003f22. Grabbing mario's position every frame if it different from the last frame, then handing it back to the editor is enough for a very simple version.

other things to consider:

• if they test for a long time, the position grabs will start using a lot of memory. perhaps store only the x most recent positions
• I really doubt it, but might slow down the emulator for some slower machines.
• position is good enough for a very simple version, but it'd be nice to grab which way he is facing as well as his current animation frame for more accurate ghosts