Make private functions private by adding a _ at the start

[niccokunzmann]

icalendar allows importing everything that is designated for the public API with from icalendar import *. Everything that is not mentioned in icalendar.__all__, is private. As such, the convention is to prefix those with _.

• rename all private functions and methods to start with _
• Rather create a small Pull Request several times than a big one that takes ages to merge.
• check back about unclear functions/classes

See also:

• Add "Code conventions" section to docs/contribute/development.rst #1004
• Add basic jcal conversion #979 (comment)