Updates the documentation for classes named from M to P to proposal A) in http://dev-list.musescore.org/Plugin-documentation-generated-manual-td7579164.html
Properties are also sorted in alphabetical order for easier identification.
No attempt made to evaluate what to include and what to exclude from documentation: anything which is currently documented is retained. Case by case decisions can always be made.
Description of methods for `QProcess` and `ScoreView` (in file libmscore/plugins.h) are stubs.
reformatting, removing some comments that are irrelevant to plugin
framework in a way that DoxyGen should still find them, documenting the
enums and fully qualify all types that are specific to MuseScore,
