fix: improve scope handling

[w-lfchen]

Description

i hate state.

this pr aims to close #992 and close #794
there are architectural issues which cause the behavior described in the issue

basically, the CustomWidgetInvocation.scope field is only set once and then used for the scopes of children widgets. since it is never updated, it will never change, but this is needed when variables update.

Additional Notes

this should be tested well to prevent introducing a regression here.

if you are reading this, please test this with your configs and report whether the changes broke anything or if everything still works

Checklist

• I added my changes to CHANGELOG.md, if appropriate.
  i don't think this is noteworthy enough
• I used cargo fmt to automatically format all code before committing
• clean up warnings that result from the changes

[w-lfchen]

initial idea: register a new scope for the children, the name might need adjusting
i've pretty much copied this from the build_loop_special_widget function

[w-lfchen]

currently, all children are in the same scope.
it might be better to create a new scope for every child

[rtoijala]

Very nice to see such a quick response :). I finally had time to test this MR.

This breaks at least the following case:

(defwindow broken
  :monitor 0
  (widg :v "foo"))

(defwidget widg [v]
  (wrap
    {v}))

(defwidget wrap []
  (box
    (children)))

Trying to open broken fails with this message:

INFO  eww::app        > Opening window broken as 'broken'
Action failed with error: failed to open window `broken`

Caused by:
    Variable v not in scope

Without the changes in this MR, the window opens and displays "foo".

[w-lfchen]

@rtoijala thank you so much for your testing!
i've updated some things (everything here is pretty hacky), and now your example works!
the following config seems to work as well (might be of use as a starting point for you):

(defpoll var
  :interval "1s"
  :initial '[0]'
'echo [0]')

(defwindow broken
  :monitor 0
  (box "test"
    (widg :v var)))

(defwidget widg [v]
  (wrap
  {v}))

(defwidget wrap []
  (box
    (for dummy in var
      (box
        (children)))))

please try to break things again, and thanks again for your cooperation!
by the way, if you run eww logs with a config containing a variable that is periodically updated, you should see how many handlers are registered for it. if that number grows indefinetly, something is wrong (but it should also throw the deallocation message).
this might be of use in your attempts to break things as well (will obviously be removed before merging)

[w-lfchen]

i'll have to see whether i can investigate #675 as well
it doesn't seem to be directly related to this issue, but closing that one as well with this pr would be neat