Move schematic loading to separate thread.

[gameboys245]

When i am loading in a large schematic Windows auto-closes mine-imator for not responding. This can be fixed by moving the schematic loading to separate thread that reports back to the main thread. This way the program can continue to respond and load schematics.

[Nimikita]

Mine-imator uses GameMaker which compiles single-threaded games/apps. While through external functions multi-threading is semi-possible, it's limited in what data the program can retrieve from other threads.