Content Model: Fix editing with default format issue

[JiuqingSong]

This changes fix several issue related to default format and content model editing.

1. When delete everything from PickerPlugin, we will let browser/other plugins (for Content Model, ContentModelEditPlugin) to delete the element, to avoid leaving an empty DIV/SPAN that cannot hold focus.
2. When delete/backspace when there is nothing to delete, return isChanged=true from deleteSelections so that outer function will prevent default behavior of delete, to avoid delete last DIV/SPAN and cause an empty editor
3. When type directly under editor content DIV, set a pending format as default format then ContentModelFormatPlugin will apply this default format to current segment
4. applyPendingFormat will convert current implicit paragraph to non-implicit, so that it will render DIV element to let it hold focus. This can fix the problem when type directly before/after a table, then a new paragraph will be created.

Related bugs:
#203168 font changes after I delete an @name
#195796 When writing 2 lines below a loop component, the font style of text change