Feature Request: New buttons in editstyle.ui

Based on feedback elsewhere I have added 3 buttons to the bottom row of buttons in editstyle.ui:

Reset to MuseScore Defaults - resets the current score's style to MScore::baseStyle()

Save as Default - saves the current styles as the default styles, creating/writing a file named default.mss in the user's Styles folder

Save as Default for Parts - saves the current styles as the default styles for parts - filename: part.mss

The appropriate fields in the preferences are updated.

Here is the PR: https://github.com/musescore/MuseScore/pull/4456
It has PDF documentation attached.