PR: Use menu and item identifiers to add items to the main menu plugin

[andfoy]

Description of Changes

This PR updates the Main Menu plugin methods to exclusively use string identifiers as opposed to using object references when adding items to a specific menu and/or before a given menu item. This is a breaking API change.

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes #

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: @andfoy

[pep8speaks]

Hello @andfoy! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file spyder/app/mainwindow.py:

Line 72:1: E402 module level import not at top of file

Comment last updated at 2021-08-17 21:46:48 UTC

[ccordoba12]

This looks pretty good, thanks @andfoy! Besides the small suggestions I left for you, I also think you need to change the places where we use before to move from actions to id's (like you did for menus).

A couple of examples are:

spyder/spyder/plugins/application/plugin.py

Lines 137 to 148 in 62dc10e

	if shortcuts:
	from spyder.plugins.shortcuts.plugin import ShortcutActions
	shortcuts_summary_action = shortcuts.get_action(
	ShortcutActions.ShortcutSummaryAction)
	for documentation_action in [
	self.documentation_action, self.video_action]:
	mainmenu.add_item_to_application_menu(
	documentation_action,
	menu_id=ApplicationMenus.Help,
	section=HelpMenuSections.Documentation,
	before=shortcuts_summary_action,
	before_section=HelpMenuSections.Support)

and

spyder/spyder/plugins/tours/plugin.py

Lines 61 to 69 in dc3260b

	docs_action = self.get_action(
	ApplicationActions.SpyderDocumentationAction,
	plugin=Plugins.Application)
	mainmenu.add_item_to_application_menu(
	self.get_container().tour_action,
	menu_id=ApplicationMenus.Help,
	section=HelpMenuSections.Documentation,
	before=docs_action)

[ccordoba12]

@andfoy, I did a manual check and some entries are missing:

Debug menu: List breakpoints is missing:

This PR:

5.1.1:

---

Help menu: The first section is missing

This PR:

5.1.1:

[ccordoba12]

Thanks @andfoy! Looks good to me now.