Add keyboard shortcuts to main menu.

What does this MR do?

1. Adds global shortcut for every item in the menu
2. Shows those shortcuts when the menu is shortcutted to.

Are there points in the code the reviewer needs to double check?

1. Make sure it doesn't conflict with any other shortcuts.
2. Make sure it works everywhere.
3. Notice that the keyboard shortcuts for the menu only show up when you press n.

Why was this MR needed?

People want the menu to be pinned. This is a happy medium.

What are the relevant issue numbers?

Fixes: #30036 (closed)