rename mode structure method: expand_char -> prefilter
s:DiffTable.remove_map() wrongly removes map if it is called twice
remove table.initialize(). add table._from_file flag.
improve/fix s:AbstractTable_load()

Improve:
* Check if any of base tables have all mappings which self has.
* Cache loaded result.

Fix:
* Base's base table is not loaded.
merge s:RegisteredWord to s:Candidate
cosmetic fix: move eskk#dictionary#_candidate_identifier() function
remove old comment
write debug log file each times 20 keys were pressed
do not call s:handle_popupmenu_keys() if g:eskk#enable_completion == 0
fix error: undefined variable
remove g:eskk#debug_out, eskk#register_temp_event()