Remap magrittr keyboard shortcut

This PR remaps insertion of the magrittr pipe operator to META + SHIFT + M (CTRL + SHIFT + M on Windows). The key `M` should be mapped unambiguously across international keyboard layouts (double-checked with Danish, German, French)

I didn't place this in `Commands.java` because AFAICS that forces insertion only in the current source document (and can't be inserted in the console window). I imagine this is also why the ` <- ` insertion shortcut is not in there.