JMenuItemBuilder: Replace constructor parameter KeyCode with int (mnemonic)

[frigoref]

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[frigoref]

@DanVanAtta: I would be targeting for the following and would like you to recheck this approach before I continue.

1. Separate mnemonics (menu navigation started with (Alt) from shortcuts (global/local)
2. Menu items should use KeyEvent wrapped local enum Mnemonics of the class containing the menu instead of KeyCode/KeyStroke.
3. Each menu level might have its own local enum Mnemonics to allow uniqueness per level.
4. KeyCode should be used for shortcuts (global and local).
5. KeyCode constants should be renamed to indicate the usage rather then the key combination to avoid reusing the same global shortcut (which lead to issue JMenuItemBuilder: Replace constructor parameter KeyCode with int (mnemonic) #13944 ).

We might want to check together some specifics as well.

[DanVanAtta]

private final int mnemonicCode;

Why not store this as a KeyCode?

KeyCode is meant to be a type-safe wrapper around KeyEvent.VK_...
The purpose of swingbuilders is to produce type-safe APIs. Which is to say, to get away from using things like int to map to keycodes.