Custom icon set for FGD entities

[LogicAndTrick]

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This requires a few changes:

• Custom set of (PNG?) icons for common HL entities (and get rid of the copied-from-Hammer sprites)
• Automatic setting of sprite icons when a sprite behaviour is not defined on an entity, or the sprite is missing
• Mapping *.spr sprites to *.png to unify file types
• Still keep some way to render sprites if they are defined in the FGD
• Search the game path for sprites and render them if they exist?