[Input-Events] 多数の更新

move input events spec to its own repository

remove unused images

add acknow. to editing overview page

add note on using getTargetRanges() to find out how many characters w…

remove outdated explainers and add directly to overview page and specs

remove cancelable attribute, related to #137

add table for data/dataTransfer overview, relates to #139

set dataTransfer=NULL for insertText beforeinput event, relates to #139

set dataTransfer=NULL for redo/undo beforeinput events

moved transferData to dataTransfer, related to #131

Author: triple-underscore