I just noticed that the main menu editor on Raspbian allows me to remove the main menu editor from the main menu. But if I removed the main menu editor from the main menu using the main menu editor, I could of course not use the main menu editor anymore to get the main menu editor back into the main menu.
So how would I get it back after disabling it?
In the console, the command alacarte will start the main menu editor.
If even the console is not available, there's the executable file /home/pi/.local/share/applications.desktop, which appears as "Main Menu Editor" e.g. in the PCManFM file browser.
A completely failsafe way, even if the menu is completely destroyed, should be to create a new text file with the .sh ending on the desktop, give yourself execution permission in the file's properties, write alacarte in it and execute it.