Each key has KEY (typedef of U8) represantation which is paired with text version of key.
To get strings corresponding to keys LLKeyboard::stringFromKey() is used.

- Added translation of keys into LLKeyboard::stringFromKey().

- Added corresponding strings into strings.xml.

- Changed LLTrans::getKeyboardString() so that it correctly process key names that are not found in strings.xml(returns English name instead of "MissingString").

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/845/

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branch : product-engine