Normalize asset names

[Laupetin]

Currently when using indirect asset reference it can happen from time to time that assets are not entirely lowercase.
This makes it so assets cannot be found, e.g. viewmodel_USPMGS_ADS_fire does not match the actual asset name viewmodel_uspmgs_ads_fire.

Therefore it is required to normalize the asset names the following way:

• Assets have all lowercase names
• Assets use / instead of \ as path separator (see #82 )