PR: Move PluginMainContainer and PluginMainWidget to their own modules

[ccordoba12]

Description of Changes

• Those classes are perhaps the most important ones of our new API, so I think it's better to move them to their own modules so they are easier to find and use.
• I also took the opportunity to remove unused imports in a lot of places.

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:

[pep8speaks]

Hello @ccordoba12! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file spyder/api/widgets/main_container.py:

Line 143:23: W292 no newline at end of file

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

Line 50:1: E402 module level import not at top of file
Line 52:1: E402 module level import not at top of file
Line 53:1: E402 module level import not at top of file
Line 71:1: E402 module level import not at top of file
Line 85:1: E402 module level import not at top of file
Line 87:1: E402 module level import not at top of file
Line 93:1: E402 module level import not at top of file