Add keyboard mnemonics

[daguej]

This adds keyboard mnemonics to the default menu item labels.

Notice the underlined letters, which represent shortcut keys for each menu item. These accelerator keys match the ones Chrome uses in its context menu.

Windows hides the mnemonic underlines by default, but the shortcut keys still work. Users can see the underlines by holding Alt while right-clicking, or by configuring their system to always show underlines in Settings > Ease of Access > Keyboard.

Linux behavior varies.

This will have no effect on Mac, which does not support keyboard mnemonics in menus.

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This also fixes a bug affecting all platforms: the mnemonic metacharacter (&) was not escaped when copying selected text into a label's {selection} placeholder, so any ampersands in the selected text disappeared from the menu (and potentially set an unwanted mnemonic).

[sindresorhus]

What about Save Image and Look Up?

[daguej]

I missed Save Image because I was comparing with the Chrome context menu, which does not have that option (only "save image as"). I'll make it m since it does not conflict with any Chrome items.

Since Look Up is only supported on Macs, there's no point in adding a mnemonic -- MacOS does not support or display them.