Redo asset loading, add a proper loading screen

[axiand]

Introduces a substantial overhaul of the way asset loading is done:

• Split up the code into much smaller functions, trying to simplify things where relevant
• Added a new splash screen/loading screen at the startup scene, letting the user know the status of asset loading (rather than sitting at a blank screen)
• Mods are now loaded one by one based on a queue system rather than everything being in one place.
• Hot reloading now only takes into account mods that have been changed, leading to a much faster experience,

Additional features:

• Added an option to toggle hot reloading on file change
• Users may now press F5 for a manual reload or Ctrl+F5 for a manual full reload

Things left to do:

• Test more thoroughly.
• Fix any bugs that come up.

[axiand]

Resolved merge conflicts

[jasminegamedev]

I think this seems good to me. It is a little bit of a critical area, so we should definitely keep an eye if people run into any asset loading bugs that come out of this, but this is probably good for now. Merging!