Add documentation about Event fields

[huderlem]

Add proper documentation at some point down the road, on what does each field of each section inside PretMap do.
For example, right now I do not entirely understand things like Location or Visibility inside the Header at the Attributes Tab, or the Replacement and Property fields when seeing an NPC's data,

(originally from yenatch/pretmap#34)

[huderlem]

There is plenty of documentation now, including tooltips and an online manual.