Small fixes and improvements

• Fixed the workflow of what happens after you finish playing a workshop level. (now you go back to the workshop browser)
• Improved the sound fx for tower interaction. Added pleasant "pop" audio fx for tower build actions
• changed music files to mp3 format, with huge reduction in size on disk
• Fixed a bug that was causing diggers not getting spawned in certain cases
• Tower selection ring visibility was fixed. It used to disappear in some cases when relocating
• Removed some artificial delay in UI, making it feel snappy
• Fixed tab button color changes
• Fixed spike damage FX. When enemy has shield intact, the splatter FX is not supposed to be visible
• 3 new levels in Sector 2