PR: Migrate Spyder preference dialog to a plugin of the new API

[andfoy]

Description of Changes

This PR migrates Spyder preference dialog to a plugin for Spyder 5. It also adds a new API for adding configuration tabs to any page via SpyderPreferencesTab

• 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 #14537

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/plugins/preferences/plugin.py:

Line 73:17: E731 do not assign a lambda expression, use a def
Line 105:17: E731 do not assign a lambda expression, use a def
Line 163:5: E303 too many blank lines (2)

• In the file spyder/plugins/preferences/widgets/configdialog.py:

Line 158:42: E227 missing whitespace around bitwise or shift operator

• In the file spyder/plugins/preferences/widgets/container.py:

Line 34:33: E127 continuation line over-indented for visual indent

Comment last updated at 2021-01-15 22:15:02 UTC

[ccordoba12]

Thanks a lot @andfoy for this! I left an initial review for you.

[ccordoba12]

Thanks @andfoy! I think this is almost ready, except for two minor comments.

[ccordoba12]

Thanks @andfoy, great work with this new plugin!