This pull request adds support for multiple keyboard layouts. Five layouts are initially included: US, French, Italian, German and Spanish. It is expected that users contribute additional layouts (I can add a few if requested). Adding a layout is straightforward by following the instructions in KeyboardLayout.h. It only requires one file defining it, and one line added to Keyboard.h.

Context

Arduino was born in Italy and has become very popular all over the world. The USA is only one among many countries that show interest in Arduino, and yet, the official Arduino Keyboard library supports exclusively the US keyboard layout. This has caused much confusion (#7, #20, #23, #24) and user requests (#8, #21, #25, #50, #52).

In the past, requests to support international layouts have been rejected with two arguments:

1. this library should stay simple and small
2. NicoHood's HID library is available for those needing advanced features.

Regarding point 2, it should be noted that:

• For the majority of Arduino users (mostly everybody outside the USA), support for a keyboard layout other than US is by no means an "advanced feature": it is the most basic way of using any keyboard-like device.
• The HID library is very limited in its support of keyboard layouts, as the API does not support the AltGr modifier.

This pull request addresses this need while keeping the code simple and small.

API change

The only change to the public API is that the begin() method now accepts an optional parameter specifying the requested keyboard layout, e.g.

Keyboard.begin(KeyboardLayout_fr_FR);

As an optional parameter, it defaults to KeyboardLayout_en_US, thus preserving compatibility with existing sketches.

Even though calling begin() is mandatory according to the official documentation, this method has been a noop until now. In order to avoid breaking existing sketches that fail to begin(), the press() method now calls begin() if it hasn't been called before.

Resource usage

For a sketch using the default layout, the cost of this change is 100 bytes of flash and 2 bytes of RAM, as measured on an Arduino Micro. This doesn't change as the number of supported layouts increases.

Using a layout other than the default costs an extra 128 bytes of flash. This is because the default layout is always loaded in order to handle the case where the user forgets to begin() before using the library. It could be discussed whether supporting sketches that fail to begin() is worth the cost.

Limitations

Keeping this library simple and small is a must. Because of this, this feature comes with some limitations:

1. Only ASCII characters are supported. Supporting non-ASCII characters would require parsing UTF-8 input, which would add significant complexity (see, e.g. a library for the german keyboard-layout capable of sending keystrokes for ALL characters #52 ).
2. While both the Shift and AltGr modifiers are supported, using both simultaneously for typing a single character is not supported. Supporting this would not add complexity, but it would require growing the ASCII map to 16 bits per cell (as in Ability to use right alt aka. alt gr to reach all characters on non us/uk layouts #50 ), which would cost 128 extra bytes of flash.
3. Characters requiring hitting multiple keys consecutively (like dead circumflex + space → ^ on some layouts) are not supported for the same reason. They also break the simple assumption that write(character) equals press(character) followed by release(character).