Minor improvements to soundbrowser file info panel. (#3406)

- Normalize sound paths, so there are no back slashes.
- Refactored icon logic for sound mount source
- Fixed mount source checking order to detect hl2 content as such instead of as gmod content.
- Expand more info nodes by default.
- Fixed sounds paths starting with special chars being reported as 0 bytes.
- Show a different icon if the sound file is missing

Author: Grocel