Add AltGr behavior option to layout

[peteyycz]

Let me give a little bit of background for this PR, for about 3 years now we install elementaryOS on all new recruits' PCs in our company so they can get familiar with Linux and have a good time while learning.

One of my new colleagues asked if this remapping was possible with eOS and I wanted to prove that if you're using Linux and open source software, anything is possible, you just need to learn and have fun along the way. ☺️ I understand if this is not something you'd like to see upstream but I'm also looking for ways to extend this plug so we could have this package available for him, so any feedback is appreciated.

We're also having a learning session on eOS app development next week. Cheers! ❤️