Sound subtitle strings are not namespaced

This is in contrast to the sound events themselves, which are. Makes things quite confusing especially when mods are brought into the picture.

Since it'd be inconsistent to put minecraft: before the translation key IDs it'd probably be better to make them more consistent with block/item/entity keys, for example subtitles.block.honey_block.slide would become subtitles.block.minecraft.honey_block.slide