Feature: Optionally disable todo propagation to parents/children (Issue #954)

taskwiki integrates vimwiki with taskwarrior, and in doing so changes
the semantics of checkboxes a bit:

    * [ ] Install Taskwiki   |   pending task
    * [X] Install Taskwiki   |   completed task
    * [D] Install Taskwiki   |   deleted task
    * [S] Install Taskwiki   |   started task
    * [W] Install Taskwiki   |   waiting task

It's still desirable for vimwiki to automatically insert `* [ ]` on
`i_<CR>`, `o` and `O` and to syntax highlight all these five as
checkboxes, so I have this in my .vimrc:

    let g:vimwiki_listsym_rejected = 'D'
    let g:vimwiki_listsyms = ' WSX'

but it results in undesirable behaviour with task hierarchies: when I
add a new subtask (using `i_<CR>`, `o` or `O`)
or mark a subtask done, the parent's checkbox is updated to reflect its
overall completion, to one of ` `, `W`, `S` or `X`, depending on
subtasks completion. This makes little sense in taskwiki. One usually
doesn't want to touch the "parent" task in taskwarrior until the
"subtasks" are done. Setting

    let g:vimwiki_listsym_rejected = 'W'
    let g:vimwiki_listsyms = ' SX'

results in slightly less illogical behaviour, but it still assumes that
(1) all subtasks are visible (not necessarily true in taskwiki) and
(2) that it's a parent/subtask relationship, not a dependency
relationship (not true in taskwarrior, questionable in taskwiki).

This commit adds an option to disable this behaviour.

autoload/vimwiki/lst.vim

@@ -800,6 +800,10 @@ function! s:set_state_plus_children(item, new_rate, ...) abort
     call s:set_state(a:item, a:new_rate)
   endif
 
+  if vimwiki#vars#get_wikilocal('listsyms_propagate') == 0
+    return
+  endif
+
   let all_children_are_done = 1
   let all_children_are_rejected = 1
 
@@ -867,7 +871,7 @@ endfunction
 "updates the symbol of a checkboxed item according to the symbols of its
 "children
 function! s:update_state(item) abort
-  if a:item.type == 0 || a:item.cb ==? ''
+  if a:item.type == 0 || a:item.cb ==? '' || vimwiki#vars#get_wikilocal('listsyms_propagate') == 0
     return
   endif
 

autoload/vimwiki/vars.vim

@@ -506,6 +506,7 @@ function! s:get_default_wikilocal() abort
         \ 'list_margin': {'type': type(0), 'default': -1, 'min': -1},
         \ 'listsym_rejected': {'type': type(''), 'default': vimwiki#vars#get_global('listsym_rejected')},
         \ 'listsyms': {'type': type(''), 'default': vimwiki#vars#get_global('listsyms')},
+        \ 'listsyms_propagate': {'type': type(0), 'default': 1},
         \ 'markdown_link_ext': {'type': type(0), 'default': 0, 'min': 0, 'max': 1},
         \ 'maxhi': {'type': type(0), 'default': 0, 'min': 0, 'max': 1},
         \ 'name': {'type': type(''), 'default': ''},