Cleanup of .cpp files by grouping methods & operator-overloads

[ChrisBrandhorst]

Check and cleanup all .cpp files, especially Playlist.cpp, by grouping methods & operator-overloads together in logical sections.

Sections should be annotated as follows:

[code]

//---------------------------------------------
// Section description, e.g. 'Operator Overloads'

[code]

[Alxandr]

The section-description should be in plain english, no hypens-string or variable-names. Also, every first letter in a word should be capitalized.