Add a mechanism to perform conversions such as C-m/Return and C-i/Tab in a unified manner

[cxxxr]

Until now, the conversion of keys was done for each frontend, but this has been changed so that it is done on the lem side.

[sakurawald]

Tested, it works well.
It's a nice thing to unify the key-conversion in the core source. Making it easier to share the key-bindings between frontends. Not to mention a new way to emit the Escape key using C-[.