Call set("disabled") and set("readOnly") in startup so children are updated appropriately. Added test cases. Fixes #17399.

(cherry picked from commit 4f2b949cfcf45f59628484f704b04989d5586fde)

Conflicts:
	form/CheckedMultiSelect.js
Move disabling code to addOptionItem. Fix disabling logic in checkbox menu items and template. Fixes #17399.

(cherry picked from commit ead78ed67c4ee46c953adf95d72a63acbcc945b8)

Conflicts:
	form/CheckedMultiSelect.js
	form/resources/_CheckedMultiSelectItem.html
	form/tests/test_CheckedMultiSelect.html