PR: Several fixes related to the tabify plugins functionality

[ccordoba12]

Description of Changes

• Fix tabifying external and internal plugins (e.g. Spyder-notebook and Debugger) that are loaded after Spyder runs for the first time.
• Move methods related to tabifying plugins and registering custom layouts from MainWindow to the Layout plugin.
• Fix error when trying to restore visible plugins that are no longer available (this error was reported by @dalthviz).
• Show visible external plugins when switching layouts. For instance, if Spyder-notebook is displayed in the interface and the user switches to the Matlab layout, then that plugin is displayed again (it was not before).
• Fix TABIFY class attribute for the Pylint and Profiler plugins.
• Reorganize code a bit in the Layout plugin.

Affirmation

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with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

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[dalthviz]

Seems like an issue on with the test_programmatic_custom_layouts related with:

• configparser.NoOptionError: No option 'layout_testing layout/size' in section: 'quick_layouts'
• AttributeError: 'MainWindow' object has no attribute 'tabify_plugin'

Maybe with the changes made the layout_testing layout doesn't get loaded @ccordoba12 ?

[ccordoba12]

I think I fixed that problem in my last commit.

[dalthviz]

Thanks @ccordoba12 ! I left some comments regarding the code to register custom layouts (maybe could be worthy to be moved to the Layouts plugins), and also the possibility to addof the error flag to the plugins get_plugin method instead of using the mainwindow version. Other than that this LGTM 👍

[dalthviz]

@ccordoba12 I think that some tests fixtures that mock the MainWindow will need an update too (like for the Pylint plugin tests)

[ccordoba12]

Yep, I just did it.