PR: Make the switcher a plugin

[AngelaRemolina]

Description of Changes

• I transformed the File Switcher widget into a SpyderPluginV2. However it is not working yet, because the old widget worked in the editor with a manager called EditorSwitcherManager and the methods that this manager handles are not migrated yet.
• Skipped switcher tests until migration is finished

What is left to do

• The switcher plugin needs a public API that contains the methods of the EditorSwitcherManager
• The order of the file menu got mixed up when the switcher action section was added. I tried to fix it with the before_section parameter in each section, but it didn't work. There might be a bug in the way the menus are getting ordered:
  

  spyder/spyder/api/widgets/menus.py

  Line 220 in ad14a7b

  def _add_section(self, section, before_section=None):

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

Issue(s) Resolved

This is necessary to solve #3860

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: @angelasofiaremolinagutierrez

[ccordoba12]

Hey @angelasofiaremolinagutierrez, your code looks really good and almost ready to be merged. Thanks for your work on it!

Most of my suggestions are purely stylistic. In case you haven't worked with Github suggestions before, please take a look at this page to understand how to apply them in a single commit.

[ccordoba12]

Besides my comments above, you also need to add __init__.py files to the widgets and tests directories like this one:

https://github.com/spyder-ide/spyder/blob/master/spyder/plugins/plots/__init__.py

That way Python will recognize those directories as proper modules.

[dalthviz]

Thank you for all the work here @angelasofiaremolinagutierrez ! Left some suggestions to hopefully fix the failing tests. The suggestions mainly focus on:

• Some extra code that got readded and should be removed related to editor actions that were being done inside the mainwindow but that are not there anymore (create_edit_action related things).
• Some extra changes in the config related files to make the switcher shortcuts work.
• A way to keep the switcher working by creating the EditorSwitcherManager instance
• Some code style things (extra readded comments to remove, a white space, etc).

If you have any question regarding the suggestions let me know 👍

[ccordoba12]

Closing and reopening to restart our tests.

[dalthviz]

After checking for some time I think I found a way to workaround the issue with the sections order in the File menu @angelasofiaremolinagutierrez @ccordoba12 !

Left some comments to address that. Also, left some code style suggestions that I found while checking the code.

Finally, checked locally and seems like everything is working as expected 🎉, so other than the above this LGTM 👍

[ccordoba12]

Looks good to me now. Thanks @angelasofiaremolinagutierrez for your great work here!