Refactor Smart fades

[MarvinSchenkel]

This PR refactors Smart Fades into separate classes, such as Smart Fades and Filter classes.

What was done:

• Introduced base classes for Smart Fades and Filters, so that we can easily re-use filters and add new Smart fade modes
• Added the following filters:
  • TimeStretch
  • Trim
  • FrequencySweep
  • Crossfade
• Renamed the mode 'smart fades' to 'smart crossfade' to better represent what is is doing
• Refactored standard crossfade and smart crossfade to use the new base class