PR: Add methods to simplify access to our config system to MainWindow

[ccordoba12]

Description of Changes

• Small refactoring whose main purpose is to add get_conf/set_conf methods to MainWindow in order to simplify how we access config options on that class.
• Move __init__ to be at the beginning of MainWindow (I think it's odd to have it in the middle of the class).
• Move the Qt methods we override on MainWindow to be on their own section.
• Add more section separators to MainWindow to make easier to navigate it.
• Remove old Mac stylesheet because it's no longer needed.

Notes:

1. I did these changes while working to fix another bug. I preferred to break that into two parts to facilitate reviewing it.
2. I think we should avoid using CONF because it's cleaner and easier to read to have methods that call it behind the scenes.

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: @ccordoba12

[pep8speaks]

Hello @ccordoba12! 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 83:1: E402 module level import not at top of file

Comment last updated at 2022-08-05 18:24:37 UTC

[ccordoba12]

Let's hold this PR until PR #18778 is merged because it touches the same code.

[ccordoba12]

@dalthviz, scratch my last comment because Ryan agreed to leave PR #18778 for Spyder 6.

[dalthviz]

Thanks @ccordoba12 ! Left a comment regarding the creation of a method to access shortcuts in a similar manner as to get and set configurations. Other than that this LGTM 👍

[dalthviz]

👍