Add an UI item to send difficult key combinations to the guest VM

> Please browse the existing issues and filter by 'enhancement' to make sure this has not already been suggested!

These are related: #4643, #4457, #3413 but I still thought adding an UI item deserves a chance to have its own dedicated issue. Please think of the ~~children~~ novice users / macOS switchers!

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Please (re)consider adding a convenient way to send "difficult to achieve on mac keyboards" keypresses to the guest OS, without needing to clunkily capture the keyboard (**and** having to know/guess that `Option` obviously(?) means `Alt` inside other operating systems, and also know/guess that you also probably have to randomly press `Fn`). These things are *not* obvious to everyone.

I think this would be very convenient and useful, especially to people who don't happen to already know those very macOS specific things (eg. people switching to macOS from Windows).

So I'm asking for a menu that contains some of these, or preferably all of these:
- `Ctrl-Alt-Del`: useful in Windows, also in Linux
- Useful in Linux VMs: `Ctrl-Alt-F1` - `Ctrl-Alt-F3` (switch between virtual consoles), `Ctrl-Alt-F7` - `Ctrl-Alt-F8` ([switch back to X11/Wayland](https://devicetests.com/revert-ctrl-alt-f1-ubuntu)), (also possibly `Ctrl-Alt-Backspace` to kill X11)
- Possibly others?

I think these are currently unnecessarily difficult to achieve using a restricted MacBook keyboard (not something a macOS beginner would know how to do).

Here's an example from GNOME Boxes which has this feature even if the UI is otherwise very minimal: