UI: Refactor main window menus

[cg2121]

Description

This makes it so we don't have to manually delete each menu each time they are shown, they are now created on demand, saving some memory. This also moves the menu code to a separate file.

Motivation and Context

The menu code has always bothered me

How Has This Been Tested?

Clicked through each menu to made sure they still worked

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[cg2121]

I have updated the code to remove the sender methods from the menus.

[cg2121]

Updated to adapt to the new frontend structure.